How much to charge for a preschool art "class"?

The local preschool MOMS want to have a fun paining session with their kids, I have the facilities and tools, the canvases will run 3-5.00 each and the paint about 2-3.00 for each child. I was thinking 20.00 per child-should it be 15.00? And certainly not over 6 children at a time. I work with local elementarys and have done murals with 200-750 students during the course of a week but this is far different. Its about Mommy/Child time and a product they can enjoy and hang up later etc. So-1-2 hours tops on time also.

Thanks in advance-just looking for ideas.