Don't want to play the devils advocate... but if you _must_ sit next to a person in need... you have to reserve the seats. Doesn't matter if it's a child, a dependent parent or a colleague that you need to run through an upcoming presentation with.
Currently, it's just the case that parents get a discount on the seat reservation fee.
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.
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.
Currently, it's just the case that parents get a discount on the seat reservation fee.