Personally, I believe it is safe to fit children in contact lenses if:
1) they are properly fit
2) they give their eyes a few hours of breathing time a day and a day or two a week
3) they have an updated pair of glasses that they like
4) they replace their contact lenses and cases as prescribed
5) they are independent of their parents in terms of insertion, removal, and care of the lenses
My experience is that children are (generally) more responsible in caring for their contact lenses than adults are. Perhaps this is because children tend to trust the doctor's advice.