Originally Posted by
pdisme
Amazing that Southwest solved this issue over a decade ago and here we are in 2023 and Delta still can't do it. With Southwest, if you're booking a minor, the system asks you for the reservation & name of the accompanying person, then you check out like normal. From that point, you cannot cancel the adults reservation online without first logging in as the minor and cancelling their reservation, but you can also take that step if you needed to, all without human assistance.
Alaska Airlines also lets you book an adult ticket, then later return to the website and add a new ticket for a child that you can link via the adult's confirmation code. I've used this many times booking for myself and my two kids, only one of whom is no longer a minor child. For various reasons, I book separate flights for each person (and each direction) far more often than I book round trips including multiple travelers.
Alaska offers seat assignments; I don't fly with Southwest because of how important it is to me that I have predictability regarding my seating position.
Screen grab of Alaska website ability to add child booking to existing adult reservation
Legacy IT systems may require effort (expense) to add this functionality, but clearly it is possible if the will is there.