I use Mehron professional face paints. The colors are vibrant and long lasting, yet they wash off easily with soap and water. If some of the colors are more stubborn, rub them with a little baby oil on a cotton ball.
Although not labeled hypoallergenic, the ingredients the paints use are all low risk for allergies. Here's a link to an article about Mehron makeup: http://mehronblog.blogspot.com/2008/12/what-is-hypoallergenic-makeup.html
If a child has a runny nose or a rash on their face, I will offer to paint their hand or arm instead.
I can paint about 10-12 full face designs in one hour or 15-20 cheek/hand designs.
I bring a menu board or notebook with pages of designs for the children to choose from. I am also willing to take suggestions for new designs.
With the parent's permission, I may request to take a photo of a child's painted face or hand to add to my notebook or gallery of designs.