I love to set up my easel at outdoor festivals and let kids (and even adults if they want!) try their hand at painting. It's the only opportunity most of them have ever had to paint in oils. They get such a charge out of the experience, and I enjoy giving them some simple instruction and letting them go at it. Even if it's a painting I intend to finish to show, there is nothing I can't fix later such as an off color or value.
Some kids get so into it their parents have to practically pull them away. But most are happy to paint for awhile and turn the brush back over to me. Parents love it because their kids get to try something new, and they often take a bunch of pictures to remember the experience.