Our youngest always wants a window seat, and it works best for multiple reasons: keeps him contained, entertains him, fairly cozy for sleeping. Usually only one parent sits next to him, the other sits separately to get some downtime if we're all traveling together. With the older girls, they've usually liked to sit separate from us since they were about 10 and 11. We let them choose their own seats. It usually works out fine, but we have to be systematic in that case about who is responsible for what carryon luggage.