the hand crank is a great idea. you can adjust the pulley sizes so the handle can be cranked at an appropriate adult/child speed, but result in prototypical rolling speed. if you want vids of coal or wood chip roll-overs, i can get them easily (i work for the RR).
Be proud, I've almost become convinced to scrap powering the Walthers dump, but for one idea. SO here I'm going to bounce it. What if instead of a motor, we either rig up a side crank, like the things on music boxes for real human hands, and a large sign on the side that said "TURN SLOWLY", or the same large sign and a rubber glove that is used to thumbwheel the rotary around. What say you?
-Morgan