My kids (age 11) are taking classes in face painting from a lady who does clowns, face painting, balloons, etc. If she can teach a half dozen 9-12 year olds, well, then yes you can do it. I tried my hand at it one class when they were short a kid (they do each other in turn), and except for the fact that I'm absolutely no artist, I did fine when I stuck to shapes and geo-designs, but I failed miserably when trying to make an animal or something recognizable, lol.
If you buy QUALITY paints, not the kind you can get at Michaels/Joanns, then you are buying paints that have antibacterial properties in them already, and are dematologist approved, meant for cosmetic use. The cheap stuff says "non toxic" and that simply means that you can eat it and not be harmed....the quality stuff is too.
The very first class she demonstrated the differences (on herself).....the quality paint wiped off with a little bit of soap and water....the cheap paints she scrubbed and scrubbed and still had discoloration. She also painted one back of her hand with quality and the other hand with cheap....then we went outside in the heat (it's over 100 degrees here) for about 5 minutes......the cheap stuff was running and would be staining your collar....the quality stuff was staying in place.
She said, if you still decide to do the cheap stuff, then you should have a brush for each kid and should wipe the top of the paint off with a paper towel.
Regardless of which paint you use, she said that any child with open sores, including popped zits, should not be painted. She suggested doing that child's hand or arm instead of their face. It's for both the safety of the paint and the child's face, because either paint can sting on open cuts.
Anyway.....there are plenty of books to give you ideas for designs, check your library. You could probably also find plenty of designs and instructions online.
Quality paints can be ordered online.....some brand names to consider: Snazaroo, Paradise Professional, Grimas