Anyone using dye powders needs to be extremely careful with them. You do not want to expose children to the powders at all; they are toxic if breathed in, if carried to the mouth, etc. They are safer for children to handle after they are mixed with water. In classes, even as adults, we were always required to wear masks and gloves when handling dye powders and we were very careful to avoid spills.
Here's a website that explains safety precautions:
http://fabricdyeing101.blogspot.com/...th-safety.html
Personally, if working with children, I would opt for the tie-dye kits.