This is typical for me when my family travels. Our 7 year old gets split off and then marked as an unaccompanied minor, requiring the at-airport check-in, and the big yellow ! triangle at the kiosk, requiring agent input.

I think - but can't swear to it - that once I called in and asked the agent to "re-link" the kid's reservation to her mom's, and this permitted me to do on-line check-in. (Since they're always checking bags, on-line check-in doesn't really matter too much to us.)