We have always used the car seat for our child when we’re travelig with it and she has a seat (from first flight at 3 months to now at 4 years). When we didn’t buy her a seat, we always asked the gate agent if there was an empty seat next to us and if we could bring the car seat on board; the answer was yes about 50% of the time. It’s both safer and more comfortable, although never required. She definitely sleeps better in the car seat.

Note that for take off and landing, a lap infant must be held, not in any sort of carrier. So if she was nicely asleep in a carrier, we had to take her out for takeoff or landing, often waking her. Car seat avoids that issue, though of course we have to take her out for boarding/deplaning.

(Actually now we don't use the car seat in some cases; she’s big enough that she sometimes kicks the seat in front of her, and we think without really having checked the evidence that the safety benefit is smaller at this age. Obviously we try very hard to enforce the no kicking the seat rule, but she never kicks the seat when she can’t physically reach it! But this is not relevant for an 18 month old.)

But no airplane that flies ORD-SNA has this issue (and it’s not an issue in coach on any AA plane); ORD-SNA is narrowbody only, and all AA narrowbodies have only forward-facing seats. Therefore, tpvn can use the car seat if they wish, assuming it’s approved for aviation use.