Decorating clay flower pots. Anyone do this?

My MOM'S Club is doing this for a spring party, or more precisely, having the kids do it. They each will have a pot to decorate and plant stuff in. What is the easiest and least staining method? Tempura paint? Is another kind better? Can you use markers? What about washable markers...bad idea since it's a clay pot? I don't know if it's more important to use a washable medium or to let them kids get stained so the pots are more lasting. We will not (I don't think) be putting polyurethane on them at the party, as they will not have time to dry. That is what got me thinking about markers. They dry much more quickly, but I don't know how well they will take to flower pots. Anyone have any experience? TIA!

Here's a link to a stencil site that does lots of clay pot painting. Not sure if this helps or not. Lots of cute ideas.

http://www.stencilledgarden.com/project-pot.html

When I taught first grade I had the kids do this for a mother's day present and they used tempera paint. However, you MUST spray with poly (generously) before planting anything in them or when you have a plant in there and water it, the paint will wash away (terra cotta is very porous and holds a lot of water). Any water-soluble paint will need to be polyurethaned before coming into contact with water. We used tempera because in a school setting we had a ton of it, but know that it does stain clothing! It was a cute project, though, the kids really enjoyed it and we attached a little card with a poem about growing up and blooming, it was very cute!