Hi Chris
I also do face painting as well as magic. There is a great forum, facepaintingchat.co.uk where lots of the best facepainters in the country hang out (including the world bodypainting champions etc) so you can get loads of useful information from there.
My favourite DV for learning to facepaint is Mark Reid's Paint . It has all the basics from sponging and using a brush through to doing some really fantastic designs.
Books are great, but I wouldnt recommend letting kids choose from them, unless you are confident you can recreate it accurately. What most of us do is print up photo menus of our OWN work, so that it is an accurate representation of our work.
Start off by volunteering at school fairs, practice on yourself, your kids etc, before you start charging for it! I think Id probably painted around 100 faces or more before I considered myself to be good enough to charge. As with magic, going out and trying to do it without enough experience just leads to disappointed kids and damages your reputation.
As Dale says, Snazaroo is great basic facepaint - good value and works fine. Once youve been at it a while, you will find there are certain colours you'd want to get in different brands (or if you are a fanatic like me, you'll find you need 10 subtly different shades of every colour in EVERY different brand!)
Another good resource is
http://www.facepaint.co.uk/ the face painters association for training courses etc.
Good luck and feel free to PM if you want any more info