Are We Done Yet?
Pro tip when shooting reference photos (scrap) of kids: Don't take too long! Bored models make for awkward reference photos.
Pro tip for dealing with bored models: Bribe with ice cream.
In all seriousness, shooting reference photos is tricky enough with paid, adult models. Shooting scrap with kids comes with some additional challenges.
Most kids are extremely awkward if you try to pose them. What works far better is giving them a role to play, letting them act it out, and shooting as many photos as possible. Plan on getting 1 good photo for every 100 you shoot.
If you can find a way to set things up so the pose can happen naturally while the kids are actually playing, you will get much, much better results.
The best photos usually come after the first half-hour or so when the kids have either become accustomed to the camera or, better yet, forgotten it is there.
Take breaks as often as possible, and be extremely patient. Be ready to abort and try again another time if needed. If the models aren't into it, it will definitely come through in the photos.
And, seriously, bribe them with ice cream.
One last thing. If your models aren't your own kids, definitely make sure their parents are present and 100% on board. Obvious point, but worth mentioning.