In reply to Daniels735:
I can't give you any actual information, but gentle bodyweight and core exercises are commonly given to gymnasts from the age of 5-6 upwards.
Assuming children are going to be running around, playing on climbing frames, jumping on the furniture etc, they will be doing these sorts of exercises in a less structured (and unknowing) way anyway...there's no reason not to give them in a more structured way as well as long as you don't overdo it.
Just stay well away from any weights and keep it relatively low-key.