Ive always added mine to the reservation when booking, or called that in shortly after. But my understanding is that even though on domestic flights, a lapchild doesn’t need a ticket, they still need to be added to the PNR. Is this not the case? So you’d still need to do that at check in. Isn’t that why the app and web site asks if you are traveling with a lap child when you check in (unless that’s changed - it usually asks me) when im not. Or can someone just walk into a flight with a kid without having a carrier know about it all?

IMO, would solve a lot of issues if you were required to do it - would be easy to avoid seating mishaps (since there are different requirements on different planes (ie ER4 only have 4 rows you can pick, CR2/7 can only select C/D seats, etc.), block exit row seats (already done with kids on a reservation), allow free E+ selection online for bassinet use on international flights, etc. isn’t there also weight and balance issues if they don’t have the lap infant data?