I found that simply using cornstarch, adding a bit of cold water to make a thick slurry, then adding boiling water from an instant kettle yielded a great paint that you can thicken or thin depending on what you want it for. Thicker for toddlers wanting to just mush around in it. Thinner for using a brush and painting.
Easiest. Paint. Ever.Gather a medium bowl.
Add two tablespoons of cornstarch and 2 tablespoons of cold tap water.
Whisk until smooth.
Add boiling water a little bit at a time while whisking to get the viscosity that you desire.
Divide amongst smaller containers that you don't mind staining from food coloring.
Add a few drops of food coloring to get the desired hue. (I didn't add too much food coloring as I knew my toddlers would get it all over their faces, hands, etc and didn't want it to stain.)
Set this activity up outside on the ground or low table. Put toddlers in an old shirt and maybe just diaper. Let them paint objects, plastic ride-on toys, or paper, with brushes or not. They can eat it, since it's just cornstarch!
The biggest tip:
DO NOT let this paint dry in hair, on skin, or on any surfaces.
BUT - Make it even more fun by getting out a hose and spaying everything and everyone off.