Why does a chain keep on coming off a mini moto?

You're chain is "stretched to far". Or you need to use a wrench to loosen two nuts and then use a wrench to move your back tire backward to tighten the chain. Then re-tighten the keeper nuts you loosened to begin with. As you tighten the chain make sure your tire is not going crooked. There should be visual notches near the bolts you are tightening to make sure your tire stays straight.