my 11 year grandaughter wants/needs a lens to take... (
For the large part, ZOOMs are not macro lenses. Most aren't even close, even though they might have a "macro" designation. One of the rare exceptions is the Canon EF 24-70mm f/4L IS USM which can do .70X (almost 3/4 life size) on it's own. That is roughly triple or more the magnification possible with most zooms. It's double what the better zooms can do.
A "true" macro lens is almost always NOT a zoom. Canon makes some great ones.
One of the most compact and least expensive is the EF-S 60mm f/2.8 USM Macro
. It can do full 1:1 or 1.0X (live size). This means that on a camera like T6, it can focus close enough to render as small as an area about 13x22mm... same size as the camera's image sensor. The only problem is a 60mm lens makes for very limited working distance.... the distance between the front of the lens and the subject is only a few inches (approx. 3-1/4"). This also is a "crop only" lens, meaning that it can only be used on Canon's crop sensor cameras... not on their full frame. This is fine for your granddaughter, because hers' is a crop sensor camera. But it does limit her option to use it on a full frame camera in the future, should she ever buy one. It also will effect resale, if/when she ever decides to do so with this lens. There will be limited buyers for the lens. Most other Canon macro lenses are full frame capable, which can be used on both full frame and crop sensor cameras. EDIT: I just learned that Canon has discontinued this lens. It's no longer available new most places, but can be found on the used market. B&H Photo has one they list with "8+, moderate wear" condition for just under $300. (Last month Canon announced 24 of their approx. 90 EF/EF-S lenses were being discontinued, as they move production to the new RF mount lenses for the mirrorless cameras. I am guessing the EF-S 60mm is one of the lenses that was discontinued.)
There is also a Canon EF-S 35mm f/2.8 IS STM Macro
lens that's even cheaper than the 60mm (above) at just $350. It's ultra compact, but such a short focal length will put the front of the lens extremely close to subjects at the highest magnification. I would not recommend it for general outdoor macro shooting like you describe. It will work best for people who don't need full 1:1 magnification (such as food photography in restaurants) or are using it in a table-top studio setting or for copy work. At full 1:1 the front of this lens will be less than 1" from the subject. A neat feature of this lens is lens has a built in LED lamp to illuminate subjects. As a "ring light", it makes for pretty flat, clinical looking lighting. And it draws on the camera's main battery, rather than having a separate power supply like an accessory light or flash normally would.
Canon's EF 100mm f/2.8 USM Macro
is an older model that doesn't have image stabilization, but is still an excellent lens at a moderate price. I recommend this focal length for most people's first/only macro lens, since it's a good compromise for a variety of macro shooting situations. It's long enough focal length that it gives reasonably good working distance, while not being so long that it's difficult to hold steady and renders extremely shallow depth of field. It is unusual among macro lenses around this focal length in that it can optionally be fitted with a tripod mounting ring (sold separately), which is a VERY useful accessory for macro photography. I've used a copy of this lens for many years. Among a half dozen macro lenses, it is by far my most used... my most "go to" macro lens. It sells for about $600 new and has been around for a long time, so used copies are pretty easily found at lower prices.
Canon EF 100mm f/2.8L IS USM Macro
is an update of the above lens that's been available for some time now. It adds image stabilization and improved weather sealing to "L" series quality levels. It also can optionally be fitted with a tripod mounting collar (sold separately). At $1300 it's one of the more expensive, but also one of the best macro lenses anyone makes. Since I use a tripod or at least a monopod most of the time shooting macro, I couldn't justify the cost of an upgrade to this lens and continued to use the above model instead. The image quality is virtually the same, so that's not a concern.
Canon MP-E 65mm f/2.8 Macro
is a very specialized, ultra high magnification lens. I DO NOT recommend it to most people because it is ONLY able to to 1:1 to 5;1 magnification. This lens can ONLY do macro and even there it can ONLY do high magnification work. Great for bugs, but not for flowers and impossible to use for people portraits or as a general purpose, short telephoto. It is also a manual focus only lens that's not easily used hand held (I mostly use it on a tripod specially set up for macro work). It comes with a tripod mounting ring (uses the same one as the EF 100mm and EF 180mm, but not the same as the EF 100mm "L") and is fairly expensive at $1050.
Canon EF 180mm f/3.5L USM Macro
is another lens that I consider to be somewhat specialized. It's a great lens that can be very useful for it's additional working distance, when shooting dangerous or shy subjects that are difficult to get close to. However, it's more difficult than shorter focal lengths to hold steady AND it tends to render very shallow depth of field, which forces one to use smaller lens apertures that in turn require slower shutter speeds. As a result, a tripod is pretty much required. It's also not a particularly fast focusing lens, so doesn't work well as a moderate telephoto for non-macro work with active subjects. It comes with tripod mounting ring and is the most expensive Canon macro lens at $1400.
Note: Only the two "L" series lenses above come with their matched lens hoods. Like all Canon lenses, the non-L lenses do not include the hood. It's sold separately and typically cost $25 to $50 (in most or all cases there are considerably cheaper third party "clone" hoods available, that probably work just as well).
In addition to the above, there are: Sigma 105mm f/2.8 OS HSM Macro
and Sigma 70mm f/2.8 Macro
that are quite good alternatives (cost new between $500 and $600 approx.)Tokina 100mm f/2.8 Macro
that is one of the least expensive at about $400, with image quality that's good, but not quite as good as some others. It's also slow focusing and lacks some features found on other macro lenses.
Just recently discontinued Tamron 90mm f/2.8 OS USD Macro
Model #017, the latest and best of a series that's been in production since the 1980s. One of the sharpest macro lenses available, it's still being made for other camera systems, but I see it's been discontinued for Canon EF/EF-S mount (I bet Tamron will soon offer it in RF mount). It sold for around $650 new and may be possible to find on the used market. There was an earlier version of this lens with OS (stabilization) and USD (ultrasonic focus motor), but not as well made or as good image quality. There also were cheaper versions with AF, but not with OS or USD.... now all only available used.
There are also a number of manual focus/manual aperture macro lenses from various manufacturers. Some of them are pretty capable, but they will be slower and more difficult to work with.
Some low cost alternatives are to use an accessory close-up lens or macro extension tubes with current lenses. Both these can make the lenses she has now able to focus a lot closer for higher magnification.
Accessory close-up lenses screw into the front of the lens, much like a filter. Think of these as magnifying glasses for the camera. There are cheap ones ($25 for a set of several) that are largely a waste of money. They ruin image quality. There are better, but more expensive ones, multi element "diopters" from several manufacturers. In fact, Canon makes some of the best of these. They've offered two different strength diopters in a few select sizes. Canon 500D close-up lens has been available in larger diameters (58mm, 72mm, 77mm) and is actually the weaker of the two types. This appears to have been discontinued, but can still be found used pretty easily. Canon 250D close-up lens is higher magnification and is still being made, though has only ever been available in smaller diameters (52mm and 58mm). These are available new for $80 to $90. If your granddaughter has the "kit" lens with her T6, it's probably an EF-S 18-55mm. I think that uses a 58mm filter, so that's the size close-up lens you'd want. There is some compromise of image quality even with the top quality Canon diopter lenses. It varies depending upon what lens they are used upon, but mage quality using these will not be as good as it can be with a "real" macro lens. Also, if the photographer has a variety of lenses for their camera they may need different diameter close-up lenses for them.
Macro extension tubes are another way to push her current lenses to focus closer and render higher magnification. These fit between the lens and the camera. There are no optics in them... so they generally have minimal effect on image quality. Different length tubes are used for more or less magnification. Also, the longer the lens focal length, the more extension that's needed to significantly increase magnification. Canon makes macro tubes that are excellent, but only in two sizes (12mm and 25mm) that are sold individually and are quite expensive. Kenko makes a tube set that includes 3 sizes (12mm, 20mm, 36mm) that make for seven possible amounts of extension, either singly or in various combinations. Costing around $140 new, the Kenko set are similar in quality to the Canon (just one of which can cost almost as much).
Easy to use, macro extension tubes can be fitted behind any lens that can attach to her camera. Most often they're used with lenses in the 50mm to 200mm focal length range (on crop sensor camera's like hers). There are plenty of exceptions, but those tend to be the most practical. So she could use them with her 18-55mm lens set toward it's more telephoto focal lengths. Or, if she has one, she could use them with an EF-S 55-250mm or EF 75-300mm lens.
Personally I have both close-up lenses and macro extension tubes. I ALWAYS have some tubes in my camera bag, just in case. They;re versatile, widely useful, inexpensive, easy to use and don't take up much room or add much weight. I only have one size Canon 500D, for use with one specific lens. I rarely use it.