Age group(s) for this birthday party activity:
Preschool and Younger
Elementary School
Middle School
High School & Beyond

Cost of birthday party activity:
The book, by editors of Klutz press (known for sturdy Take it With You activity books of all sorts) details 83 designs, from full face makeup to simpler "cheek art" designs. It retails for $19.95, but can often be found for less, from e-tailers like Barnes and Noble and sometimes 'bricks and mortar' stores, too. **Paint IS included, in a palette with brush! You should be able to get more than one party out of that, too. The photos, by Thomas Heiser, are very clear and helpful.

Barnes and Noble lists 3 other titles for face paint books, too.

To find the paint itself during the off-season, consult costume rental shops, party goods retailers and e-tailers. Do Not consider tempera paints; although non-toxic, colorful, and cheap, I am told that when it dries, it flakes and itches.
While the kits may seem pricey, some are designed to Last awhile. Read the labels, looking for face-friendly products.

Birthday party planning information:
Get your book and/or paints ahead of time, and Practice a little before your party/event!

Other suggestions or comments:
Wear an apron. Have shade and/or sunscreen available to painters (like a canopy or umbrella table). Have fun! Take lots o' pictures!