Our face paints are almost always easy to remove with nothing more than a warm, wet wash cloth and maybe a little bit of baby shampoo. Like most make-up, sometimes dark or bright colors can leave a faint shadow on the skin for a few hours after washing. It also depends on how long they have the face paint on and what type of skin they have.
The best way to get rid of it is to gently rub it away with a cotton pad and Vaseline. Additionally, some parents choose to simply wait a few hours and wash again, in which case it usually disappears completely. You can also use baby oil or baby wipes.