Before I even had to deal with food allergies, I would routinely use rice crispies for cake decorating. As a child and teenager, I and my siblings would decorate cakes each year and enter them in the county fair. Sometimes our cakes would fall, or not rise satisfactorily, or fall apart when frosted. We found that rice crispies tended to make uniform cakes more consistently than the other cake recipes we used.
The image I've attached is of my daughter's birthday cake this year. We had a party at the park and served rice crispies! The kids enjoyed it. I found instructions off the internet for making the butterfly cut-out cake shape. You use a round cake pan and do a bit of cutting, then piece it together to make a butterfly. I used fruit roll-ups for the antennas.
To make non-dairy frosting, I find a frosting recipe, then use water instead of milk(or I use soymilk or rice milk), and then replace any margarine called for with vegetable shortening, or use both non-dairy margarine and vegetable shortening(this is more expensive, though).
NOTE: Make sure that the brand of rice crispies you use doesn't contain wheat. You never know until you check the label. Watch out for malt flavoring, malt extract, etc if you need gluten-free- this is usually derived from barley from what I understand. All the brands I'm aware of have malt in them.