Tips for painting with kids:
~ Cover more than enough area so you don't worry about a mess. Their creativity will suffer if you keep telling them to stop dripping or flinging paint.
~ Mix paint with water and give them a lot of paint. It's not fun to paint with a half dry brush.
~ Use big brushes.
~ Use a big canvas. I take photos of their paintings and let them paint over them again and again.
~ Wear paint clothes. Every artist gets messy.