Make sure the kids are wearing old clothes - this could get messy.
Pour out some paint in a shallow dish.
Let the kids dip their hands into the paint, then press them onto the outside of the terra cotta planter.
You can use several different colours, just wash the hands before jumping to a new colour.
Let the paint dry. If you wish, you can spray the outside of the pot with polyurethane to finish it off, but it is not necessary. DO NOT let the children use the polyurethane.
Plant Grandma's favorite plant in the pot.