I've been doing this for years. I save old egg cartons for paint pallets for the kids. The cardboard ones are really easy to cut - so you don't have to give them the whole thing - or if your kids are like mine, heaven forbid they share paint.
This is also a really great idea if you are hosting a birthday party or playdate with paint. You are brave for attempting - so now be cheap and let the kids' imaginations run wild!
This post is partially inspired by the group challenge Precious started at Frugal Makes Cents.