I've been following this with some interest, and frankly, it sounds pretty disgusting. Why not put the on real bikes, at the park, where there's stuff to see, and other people to talk to and play with?
Maybe set up a sort of designated path, with little check-in points, with little flags that they collect?
Or, just sit them on the exercise bike, and stand over them with a cat-o-nine-tails for "encouragement."
Good Luck! Rich