dont worry about the thinning of the skin, use the prescribed amount, children with excema do have thickening of the skin as a result of excema. if you use a moisturiser like dermol, then apply the steroid cream, unless he is scratching it should reduce.
If he is scratching it then its worth asking for some tubi-fast bandages to apply over the top which will eliminate the excema, you have to be regimented though, do it when you have to do it, and you will see results.