There's a basic requirement in commerce that products sold must be fit for purpose. That is, they need to actually do what they're supposed to do in some form. I can't sell you a flight to New York and then give you a pair of plastic wings and say that the rest is on you. I have to actually get you there like any reasonable person would assume I would given what I sold you.

Selling tickets to a small child and their caregiver and then seating them far apart is plainly not fit for purpose. They can't actually fly like that, so you've sold them something they can't use, and that you know they can't use.

If they want to charge extra to sit together, fine, but that needs to be bundled into the basic price when one of the tickets is for a small child, not presented as an optional add-on at an additional cost.