I think what you're considering and many of the subsequent suggestions have got it all backwards.
Instead of an expensive camera with a cheap telephoto zoom, for wildlife photography your niece will get much better results with a good solid, fast focusing tele zoom on a less expensive camera. I can suggest some Canon options, but am not knowledgeable enough about Nikon alternatives so will leave that to others.
Forget the 70D or 7D. Too much cost and, especially 7D, too much camera for a beginner. ( I use two 7Ds myself, and have extensively used 5DII, 50D, 30D, 10D too... plus, less frequently, several other Canon models.)
Instead look for a new/used/refurbished 60D, 50D, T5i/700D or T4i/650D. All these are very capable 18MP (except 50D, which is still a very respectable 15MP) and - most importantly - all have the same 9-point autofocus system, with all nine points the "better" dual axis/cross type. These cameras also all have more support for a beginner compared to 7D. The Rebel/xxxD models, in particular, are more friendly "first-DSLR" cameras. The Rebel/xxxD series are not quite as fast operating as the 50D/60D, which have more direct access to some key functions. The Rebel/xxxD series also use a penta-mirror viewfinder, which keeps costs and weight down a bit, compared to the true pentaprisms in the xxD models, which make for a little bigger and brighter viewfinders.
Now with a less pricey, less complex camera, turn your attention to a better lens.
I'd recommend image stabilization to help steady shots with a long lens and ring type focus drive for it's speed and accuracy. EF-S 55-250 IS Mark II or IS STM are reasonably compact, fine optically and do have stabilization (IS), but are slower focusing and 250mm is not really long enough focal length for a lot of wildlife photography.
At a minimum, EF 70-300 IS USM at about $650 would be a better choice, but still is relatively short for smaller, more distant wildlife. (IS is stabilization, USM is ring type focus drive). A more compact/portable version is the 70-300 DO IS USM (DO is "diffractive optics" ), a nice lens but it's a lot more expensive... about $1400 if memory serves.
(IMPORTANT NOTE: You may come across the Canon FE
75-300 lens while shopping, that typically sells for $200 or less... run screaming away from it. It's
not one of Canon's better efforts. The slightly more expensive EF-S 55-250 IS II or IS STM both are optically better lenses.)
IMO, one of the best deals right now is the Sigma 120-400 OS HSM (OS is stabilization, HSM is ring type focus drive on Sigma lenses). It's being blown out for around $800-900, probably a new model coming soon.
A super long focal length yet fairly affordable zoom that's been introduced relatively recently is the Tamron 150-600mm VC USD (VC is stabilization, USD is ring type focus drive in Tamron parlance) costs about $1100. It has been a big hit and can be hard to find. Most users seem to feel it's excellent up to 500mm, a little soft between 500 and 600mm.
Canon 100-400L IS USM is a really fast handling "push/pull" type zoom, very popular with birders, air shows and other situations where quick shooting is necessary. It's about a $1500 lens, though.
Personally I use a Canon 300/4L IS USM a lot, with Canon 1.4X II teleconverter when I need more reach. This is a $1400 lens (I paid $900 used) and $250 (used) teleconverter (Current Canon 1.4X III is about $450). But I have a couple 70-200s and 28-135s, to complement the prime lens. Most users wouldn't want just one prime lens.
The Canon EF 400/5.6L USM is another very popular bird and wildlife lens.... for about $1300. It doesn't have IS (stabilization), so somewhat limits the shutter speeds one can use and/or might call for a tripod or at least a monopod a lot of the time.
There are other telephotos, but they go up rapidly in price, size and weight from the above.
She likely will not want to use a long lens all the time. So you probably will also want something shorter as a general purpose "walk-around" lens. The least expensive option is an EF-S 18-55mm "kit" lens. These are pretty plasticky and not fancy by any means, but the more recent IS versions of these lenses are not at all bad optically. You'll find three versions... IS, IS II, and IS STM. They are all similar quality, the STM version is typically about $50 more and has quieter, smoother operating focus drive (though still not as fast as USM).
A general purpose, walk-around lens I often recommend is the EF 28-135 IS USM. This is an older design dating from the film days, but is still in production and very capable. On any of the crop sensor cameras recommended 28mm isn't very wide, but is a solid "standard" to moderate telephoto with very good image quality (it rivals the 24-105, 24-70 premium series lenses for image quality). Also has stabilization and fast USM focus. And it's quite close focusing, so can be used for near macro work. Best of all, a ton of them have been sold in kit with various Canon cameras, so it's pretty easy to find a practically unused one for around $200-250. (It sells new for $480.) I've got two EF 28-135s that I use as loaners and as a lightweight, versatile option when I'm hiking or biking and want to minimize the gear I need to carry (it replaces my 24-70/2.8, 70-200/2.8 and 100/2.8 macro in my camera bag, saving about 5 pounds of weight and a lot of space). I can reference several pros who regularly use the 28-135 instead of far more expensive options (Joe Farace at Shutterbug magazine is one...
http://joefarace.com/my-gear/).
So, basically, I'd suggest instead of an $1100 camera and $250 lens... for wildlife your niece will be much happier with a $500-600 camera and an $800 to $1100 telephoto zoom lens. She'll probably also want a shorter focal length $150-250 "walk-around" zoom, too, for closer subjects.