Create a messy six sided stamp by cutting out and hot gluing craft foam shapes to the sides of a wooden block. You could probably use thick felt for the shapes, I'm not sure they would last though. I used the 1.5"ish wooden blocks that I got from Barclay Blocks ages ago, and a sheet of craft foam that I just picked up from Michaels.

Having a different shape on the top and bottom seems to confuse Rebecca, the shape she's looking at is different than the one she stamps, so it might be better to make three stamp sides, and have the other three sides be drawings of the opposite stamp sides. But that is so inefficient and boring. I'm sure she'll get the hang of it. Cognitive dissonance is good!