The most effective treatment, at least in my experience, is oatmeal and ginger baths. It's not necessary to purchase an expensive boxed oatmeal bath packet. Simple rolled oats work equally well, at a fraction of the cost. Place a cup or two of oats in an old stocking. Add some ground ginger, and tie off the end of the stocking.
Older children are more likely to understand and stop scratching.
If explanations don't help, however, it may be necessary to have your child wear mittens, especially at night. Keeping nails clipped will also help. Popsicles may help soothe sore throats, and will also help get some much-needed liquid into your child. There is a vaccine available, after all. A child vaccinated before going to school would therefore face the potential of getting chicken pox again when she reaches puberty, the most dangerous time to get it.