I don't know if this helps, but I ride with my almost 5 year old, who can't ride a bike without training wheels. It took a few short tandem rides (about 3 miles total) for us to get used to riding without him throwing my balance off (we never fell over though). I tell him when bumps, etc are coming but I've given up on telling him to sit still. He/We are doing better and I want him to have fun, not listen to me harp on him. Of course I out weigh him by a lot, which helps me counter balance him.
I would rent one, you don't have much to lose if you don't enjoy it.