What if you make pony shaped sugar cookies (I'm sure there are pony cookie cutters out there) and have her help you decorate them and place them on the cake?
Fondant is technically edible... but I don't know if I'd consider it "eat-able." The pony you made is adorable... but it's an awfully big chunk of that stuff. I'm sure the little girl would wash her hands, but having big chunks of edible play doh on a cake that have clearly been formed and handled a lot by little hands might not be appetizing for everyone.
If you still want the 3d sculpted pony, what about using marzipan or chocolate clay instead?
Or... have you considered using candy molds? You could push the marzipan into a pony-shaped candy mold to get a cute pony that's flat on one side for adhering to the cake... and you could paint it with food coloring, or use colored marzipan to begin with.