When my kids were small you could purchase several types of hard plastic molds to form your Rice Krispies in. Off the top of my head we had the Mama Bear pictured below and a Dinosaur. I had forgotten all about them until a few months ago. I decided to go dig them out.
They sell all kinds of molds now days, but you can get the actual yellow bear molds on Amazon. Just use the Rice Krispie treat recipe off the box.
You could also fill this mold with cooked rice, jello, cheese, fudge, etc.