My 6 year old son and I ride with a Kazoo.

I find the single speed is good enough on typical climbs - the most important time to get pedaling assistance. But I don't recommend riding places where the adult isn't strong enough to pull the child up the hill unassisted.

On level ground or downhill, I find that I limit our speed for safety; I don't need a pedaling boost. He often just coasts.

When he gets older he might get bored coasting or spinning when we are traveling at higher speed, but I hope to have him on his own bike or upgrade to a tandem by then. At 6 he is just thrilled to be sitting up high like a big kid.