Okay, so I've figured this out! It is a feature of the old interface but not of the new one. That's why I was having trouble.
Once I signed into my son's account, it happened to show the old interface and then I was presented with a dialog box with radio buttons. This didn't appear on the new interface at all, at any point.
Then there was another section that appeared on the checkout page allowing you to specify the PNR of the linked Adult reservation.
Alaska: please fix this! The feature needs to work on the new interface too.