pithydoug Loc: Catskill Mountains, NY

Go manual set shutter enough to cover movement 1/250 should do it, Set ap to cover all the kids especially if not in the same plane and then use AUTO ISO. Trust the camera choice of ISO, after all the 850 is one hell of a camera. At worse they may have some grain but they will not be blurry and any noise can be tempered in post.

FWIW I seldom use a flash.

jerryc41 Loc: Catskill Mts of NY

High ISO and high shutter speed.
Wait till the kids are asleep.
Get a basic point & shoot with a built-in flash.

lbjed Loc: New York

What about a Gary Fong Lightsphere. Diffuses the light. Worked great for me with my grandkids. Sharp and well exposed.

Gene51 Loc: Yonkers, NY, now in LSD (LowerSlowerDelaware)

This class of device spreads light omni-directionally, but with a point source - like a bare tube flash - and eats up flash power, increasing recycle times. They produce harsh light and deep shadows, unless you are in a small room and the light is close to the subject, usually less than 4 ft.

VTdude

Stationary lighting (daylight, studio lights, etc.) is better than a Speedlite at faster shutter speeds. From experience I know anything faster than 1/250 the flash isn't quick enough and you only get a partially exposed shot.

pila

Wonderfully explained

gvarner Loc: Central Oregon Coast

Get more light, use auto-ISO, shutter at 1/400 or faster. Any one or all of these. Or maybe make a movie?

billnikon Loc: Pennsylvania/Ohio/Florida/Maui/Oregon/Vermont

OK dad, this is going to be easy. You need FLASH, YES you do. So, this is simple. Get a used Nikon SB900 or SB910, you already have a D850, so this is not expensive for you.
When you get the flash set it to TTL, this is a default setting and should go there automatically. Now, the hard part, mount it on your camera and tilt and rotate the flash head so it points up and behind you, that's right, up and behind, about 45 degrees or more. Now, use a shutter speed of at least 250 sec. in case grand kids are moving. You can shoot anywhere from 1/30 sec to 250 sec. You can adjust exposure using your exposure compensation dial on the camera or flash.
If you want to get serious, put camera on manual, say 1/60 sec. at f 5.6 with an ISO of about 400. But leave the flash on TTL auto. Now, look at your results, it the background is too dark, open up to f4 and 1/30 sec. if too lite stop down a couple, or use exposure compensation.
Bottom line, you have the camera, you must master flash next. Good luck.

gvarner Loc: Central Oregon Coast

I would add to this - for indoors, flash in TTL mode, camera in Manual mode, ISO 400, F4.5, 1/200 for shutter. The flash controls exposure on the subject, shutter controls effects of ambient light from windows. The flash stops motion blur. Try to stay in close because this technique reduces the reach of the flash. You can increase the ISO to get more reach and change the F stop to affect the DoF. If you go outdoors, set your camera to high speed sync and use higher shutter speeds to darken busy backgrounds. Try it, you'll like it.

autofocus Loc: North Central Connecticut

that's why you setup your flash/camera to HSS (high speed sync) I've shot with flash at 1/6000 shutter speed. Two minor drawbacks, being that the flash is actually pulsing the light through the shutter cycle of closing, a little less flash output. Also, it will suck up battery life quickly.

autofocus Loc: North Central Connecticut

well, a lot of folks above are saying "use flash," "flash freezes action," unfortunately, that's not always true. When flash is used you have two light sources contributing to the final exposure, ambient, and flash. Your shutter will more or less control the amount of ambient light, and your aperture gives you control over the flash. In first curtain sync, which is pretty standard, the flash will fire a burst of light when the shutter finally reaches full open, and before the second curtain starts the process of closing the shutter...often the burst might be 1/10,000 of a second. But after that burst, the shutter is still open for a short period, say at 1/60 second, and it's still letting light in, and contributing to the final exposure. If there happens to be movement by your subject in that brief period you could wind up with what's called "ambient blur." If you really want your flash to freeze the action you need to kill as much of the ambient light as possible, and you do this with a fast(er) shutter speed. Remember what I said above, in flash photography, it's the shutter that controls the ambeinet light.

amfoto1 Loc: San Jose, Calif. USA

1. Get a flash! They're great for low light situations and freezing movement!

2. Set a high ISO.... 6400 to 12800?

3. Use a large lens aperture.... f/2.8 or bigger, if possible.

4. By doing #2 and #3, you should be able to set a faster shutter speed. I'd look for 1/250, 1/500 or faster to freeze subject movement.

5. Also be sure you are using AF-C... continuous focusing mode. This is necessary to track movement. (AF-S is for stationary subjects... if you use that it focuses, then locks.... if subject is moving, they will be out of focus more often than not, especially when using a large lens aperture that renders shallow depth of field.)

6. If you 're shooting under variable lighting conditions... i.e. if your active subjects are moving in and out of brighter/dimmer lit areas... DO NOT use manual exposure mode. Your exposures will be off a lot of the time. Instead use one of the auto exposure modes. For action shooting many folks will choose to use Shutter Priority ("S") AE. With that you set the ISO and the shutter speed, the camera selects the aperture. Personally I prefer Aperture Priority ("A") AE.... where I set the ISO and the aperture, the camera chooses the shutter speed... but when shooting any sort of action I have to keep an eye on the shutter speeds the camera is choosing, to be sure that they are fast enough to freeze the subject movement. It's also possible to use Manual ("M") with Auto ISO.... where you choose both the aperture and the shutter speed, but the camera selects the ISO to use. This is actually another auto exposure mode (in spite of the M setting). Try each and see which works best for you.

RobertH

As a relatively new D7500 shooter, one thing I haven’t seen mentioned yet is the autofocus area mode. Some have mentioned setting the camera to continuous autofocus, which makes sense for fast moving kids, but that only controls how the camera focuses, not WHAT it’s trying to focus on. If the camera got set to single point, and the OP isn’t keeping the focus point (Red rectangle) in the viewfinder on the kids, there’s not much chance of them being in focus, no matter how fast the shutter is.

Thus, I’d suggest checking what area mode it’s in, and set it to Auto Area, if it’s not already, to let the camera figure it out. If you want to keep control of where the camera is focusing by using single point, then you need to make sure the focus area is on the kids when the shuttter is tripped.

To see if this could be part of the problem, turn on “Show Focus Point” in your viewer software, to see what the camera was focusing on for the blurry shots.

BTW, one advantage of AF-Single over continuous is that in AF-S, the camera won’t release the shutter unless focus lock has been achieved. The upside is SOMETHING should be in focus when the shutter trips. The downside is that it can cause you to miss shots, which is why continuous is probably better.

drklrd Loc: Cincinnati Ohio

Or how about just turning up the lights in the room. Add a new bulb to a lap that is 250 watts strong and make it a photographic blue bulb. They have LED light panels available at B&H that can do just that too. Or get two flash guns that sync with each other and point bot of them at the white ceiling so that the bounce light covers the room and then add the third flash on camera to fill in shadows. I would prefer to light up the room with photofloods and they make them in daylight type flourescents and daylight type LEDs now.

Gene51 Loc: Yonkers, NY, now in LSD (LowerSlowerDelaware)

High speed sync only allows you to use a faster shutter speed. It does nothing about the quality of the light. And given the fact that AutoFP (Nikon's name for HSS) consumes so much power and reduces working distances, adding the Fong just further reduces the power output of the flash, resulting in higher ISO and larger apertures. Also, the quality/directionality of the light when using a Fong is difficult at best.



If you like the Fong lighting, the interesting thing is that you don't need to spend $$$ to get the same results as a Fong - you just re-purpose the container that your Chinese soup came in

https://improvephotography.com/2545/gary-fong-lightsphere-collapsible-review/

Again, the better solution is often either using off camera truly diffused (medium/large softbox, umbrella, Brolly, etc) or, has been suggested, using a speedlight with a head that can be tilted and swiveled so you can bounce off the wall and ceiling behind you or two the side. Regardless of your approach, size matters. You will get softer, more flattering results when you can make the apparent size of the light source larger - something that the Fong and other devices fail to deliver.

A cheap (used on eBay) Sunpak 383 has a high GN (120 with ISO 100), and has a duration of 1/700 at full power to 1/20,000 at 1/16 power. I found a nice balance between power output, flash duration and recycle time at 1/4 to 1/8 power, in smaller "residential" settings when using it as bounce. I will often use several speedlights for more even illumination, bounced off walls and ceilings.

I like the flashdisk much better than the Fong:

https://www.amazon.com/gp/product/B00KHBZ85E/ref=as_li_tl?ie=UTF8&camp=1789&creative=390957&creativeASIN=B00KHBZ85E&linkCode=as2&tag=fstoppers-20&linkId=GEU3RN5WPHLWZNJ5

This is a nice comparison of popular light modifiers for speedlights - along with results so you can draw your own conclusions.

https://neilvn.com/tangents/review-best-light-modifiers-for-on-camera-flash/

https://www.youtube.com/watch?v=L08G2dfIwFY

For what it's worth - Neil van Niekerk is a highly accomplished and successful wedding, portrait and event photographer with an excellent blog and tons of information on how to best use portable flash. His blog is worth a look.