What you need:
brown watercolor, watered down
gold paint (optional)
Take the straw and insert it into the liquid, keeping your finger on the tip of the straw to trap the liquid inside. Pull the straw from the watercolor and let go onto the watercolor paper. (although most paper types would probably work).
For littler ones, make sure they keep the straw away from the inside of their mouth, because they will possibly try to suck through the straw.
Once they have dried, you can use the oil pastels to draw on eyes and a nose. Although, I often find they can be beautiful on their own.