Many craft recipes call for Ivory Soap Flakes. Unfortunately, the company stopped making this product in 1993.
Soap, in general, helps keep paint from staining hands, clothes and other surfaces and it also helps paint made with gritty, powdered tempera go on paper more smoothly. Those testing and developing the recipes found that flakes were preferable to liquid soap because it kept the paints from becoming too runny. In addition, some recipes need the "fluffy" consistency of the flakes versus powdered soaps like Dreft.
You can make your own soap flakes by using any pure vegetable soap and grating it with a cheese grater. Ivory soap is about the cheapest pure soap, but you can experiment with different types to get different smells and even colors. Be careful if using soaps scented with extracts like peppermint with very young children, since it seems to make them just want to eat the paint.
I find that laying the soap flat on its widest side and using the side of the grater with the long openings (2 inch slits) is more effective than using the sides with small holes.