This is a good technique not just for kids, but for making art in general. I recommend trying it with oil pastels. Also, the bottom layer need not be colored

This is a good technique not just for kids, but for making art in general. I recommend trying it with oil pastels. Also, the bottom layer need not be colored randomly, but could use a single color or maybe a limited-color palette. Likewise, the top color need not be black, but should cover the colors underneath. Lots of room for experimentation here.