What if the kid is the only one pedaling? "Come on Timmy! Daddy's not getting any older!" haha just kidding. I don't think the kid would be putting forth any useful power, but learning to shift is good. and if they get bored they will have something to play with. i would consider putting on a light or maybe a bell for them to play with. that way you can teach him to hit the bell when you're turning or when he sees pedestrians.