Another vote for now taking the young children to the bottom. Even as adults we found it very difficult (even going down) and we thought we trained for it! :)
I'd suggest you pack each backpack with plenty of water and go down no further than Devil's Garden. It's a beautiful cool spot to rest before the hot difficult climb up. Even going down 1/2 mile will give the kids and you an altogether perspective of the canyon. You don't have to go all the way down for this. Go down a short way on each trail on multiple days for a little variety. Even the Rim Trail is awesome, giving you many different views of the canyon and it's an easy one to do using the shuttle bus system. Get off and on whenever.