Removing face paint from the skin shouldn’t be a problem providing you followed my previous advice and used a good quality water based face paint. Any good quality water based face paint should come off of the skin just by using warm soapy water or baby wipes.
If you have any problems removing face paint or are left with any staining on the skin then i would seriously consider changing brands. In this day and age face paint should (and is) very easy on the skin and should not be leaving any signs of it after removing it from the skin.
Obviously applying a white base coat is a way to help any color staining the skin, but by sticking to a good water based face paint opposed to an oil based paint should avoid this even being an issue.