Flying with 2 year old on lap?

T

Wondering if any moms know the answer to this. If I buy my 2 year old their own ticket on an airplane does that mean she is then *required* to sit in her own seat (with a car seat)?

Slightly longer version of the question: I know at 2 you must purchase a ticket for children, they can’t just share your seat as a lap infant. But does that mean they MUST sit in their own seat? My 2 year old has special needs. She’s tiny (about 20 lbs) and can sit up but obviously wouldn’t be secure in her own seat alone without a car seat.

I would much prefer to seat her on my lap (she’s about as big as a typical 1 year old so it wouldn’t be difficult) even if I have to technically pay for her own seat because of her age. I’d rather not drag her car seat on board if I don’t have to. So does anyone know if a 2 year old is allowed to sit on my lap or are only kids under 2 allowed to sit in laps?

** also please I’m not looking for a debate about whether it’s safe to have lap infants on planes. It’s not. I know it’s not. It’s also not any less safe for my daughter now that’s she’s 2 than it was three weeks ago when she flew on my lap while she was under 2.**