Keep the legs short to start with. When my youngest was probably around 6, we were chugging up the north coast of France, at all of 80 KIAS, about 11 miles offshore. Sitting in the P2 seat, he suddenly decides (this is after 20 minutes of flight) that he wants a pee. P1 anguish. I had to quickly consume the rest of a 2 litre bottle of coke, and pass it in his direction!