My son loves to mix things and get messy. Finally I found a cleaner solution, both of them get entertained and it's not that messy.
We make finger paints together. It's easy equal parts of flour and water plus 1/4 teaspoon of salt and a few drops of food coloring. I'll give him all the ingredients to drop in the mixing bowl and give him a whisk to whisk it. This keeps him happy and busy twice the time. He spends