To treat a child with Chickenpox allow them to soak in a warm bath and add some oatmeal to the bath as well to help soothe the skin and Chickenpox.
Bathing in warm water helps to relieve the itchy feeling of the Chickenpox and can help soothe the child's skin making him/her feel better.
A nice warm shower can also help with the Chickenpox and then dry the child off completely by patting them dry and then apply Calamine Lotion to the Chickenpox.
Just don't put the Calamine lotion anywhere near the child's eyes.