Easy solution, just charge more for a child than for an adult, no fees needed.

Currently children <11yr get a 20-50% discount/subsidy for a seat. So just rectify it to a 100% and give the seat as a bonus instead. Everyone happy?

Honestly I would be happy if the 5x the price, and I'm a parent. I hate flying with a kid and it would let me convince the wife to drive or take a boat the next time.

I basically only fly with a kid because everyone else is willing to subsidize the massive externality I impose on them.

I have never seen this. For all flights I have flown recently, the price for a kid and an adult is the exact same.

> must reserve

With the current implementation exposed to the end customer, yes, that's required. Reserving specific seats isn't fundamental to the constraint that some people want to sit together.

Plus, the current reservation system is predatory in its own right. When booking you're dumped into a page strongly suggesting you must choose a seat, and all available options cost more than the base ticket.

Well, any half decent operator will put you next to each other and the other half at least lets you select seats during the check-in process. If that 90% certainty is not enough for you... just reserve the seats. Yes, it'll cost money, because otherwise there won't be any seats to reserve as anyone will do it.