Mix black paint and water at about a 1:1 ratio (1/2 water and 1/2 paint).
Color clay by pushing hard with oil pastels. Make sure kids are careful on delicate areas.
Kids can blend colors. You really want to encourage clay to be covered with color.
Make a ratio of 1:1 water and ink or black acrylic paint or India ink.
Paint wash over the whole clay project.
Run project under the sink. Use fingers to help wash out areas that may have to much black paint left.
Let dry.
Optional: Use a clear coat of your choice to finish the clay project.
Notes
If you accidentally take too much paint away under the sink you can repeat the process until you are happy with the outcome. You can also use oil pastels on top of the final project to brighten any spots.