I'm with William on this. Very small children obviously need to be close to parents, but aged 8 or over, they should give precedence to adults where seating on public transport is concerned.
Why? Vulnerable people get priority, and after that it’s first-come-first-serve. Unless we’re talking about an elderly or disabled person, or someone else who for whatever reason can’t stand, then my seat is my seat until I get off.
When I get on the bus, I don’t pay to stand up, otherwise I would walk. I pay to sit down and enjoy the journey. I pay for a ticket, I deserve the seat