Well, I did a long (40minutes) 10.7 km with my youngest son (12).

We rode back and forth on the strip of false flat in front of our house.

We coasted down the false flats to see who could go furthest (me of course - I'm about 40-50kg heavier than him I think) and then we would turn around and he would sprint past me. I would catch up and sit behind him til he got tired and he would declare that he managed to get up to 30kph (or whatever speed he managed that time) before he stopped pedalling.

Really alot of fun.