If the airline expect me to sit apart from my two year old the plane won't be able to take off as dd will be going apoplectic.
They could, of course, just put the pair of you off.
"You need to do this as otherwise my child will be screaming" isn't a strong bargaining position.
In reality, no airline is going to seat a 2 year old away from an adult, as it would be wildly unsafe, so speculating about what might happen if they did is rather pointless (cue accounts of how someone's cousin's aunt had it happen to them). Aside from anything else, a 2 year old cannot fasten and unfasten their own seatbelt or fit a mask, never mind evacuate. And as scrum boarding has pretty much gone, you wouldn't get to the plane and find this out anyway; the latest it would be sorted out would be at the gate.
(It would be interesting to cross-reference airlines' rules on unaccompanied minors with their seating policy: it seems hard to see how they can insist on "no unaccompanied children under X years" if the accompanying adult can be seated at the other end of the cabin).
What they might do, however, is seat two parents and two children in two blocks of two, or put a ten year old on their own, or separate a honeymoon couple, or do other things that are a pain, but aren't dangerous. That's what you're paying for, if you don't want that to happen, or money you can save, if you don't mind.