Our local museum of art offers a free children's art class the first Saturday of every month. We stumbled upon this one recently while waiting for Dean to finish up at the hospital. We learned about the careful technique required when using chalks and how layering and smudging can be used.
They have a small, but growing, children's section in the basement of the museum with some interactive art displays and exhibits. It's not quite the Treehouse Children's Museum we're used to, but we'll take it.