Is there a kid in the world that hasn't at some time owned a pet rock? I don't think I did, but I know my sister had a lovely purple pet rock. We revamped the pet rock into a slightly better pet around here - a bug!
Grab some rocks and paint for the more basic pet. If you want to make it into a fancy bug, you'll need pipe cleaners and googly eyes too.
Paint the rocks - and for a kid there is no such thing as too much paint.
Let it set out to dry. (Or, in the case of the rocks with thicker coats of color, you may want to use a hair dryer to assist in the drying process.)