It looks like United has just tightened the screws with (supposed) UMs. I nearly always travel with my kids on separate PNRs.
For the first time ever, the online-checkin system refuses to check them in:
We are unable to complete the check-in process as children traveling alone must check-in with a United representative. Please proceed to the airport for check-in.
The GS rep tells me this has changed in the last month or so. The only thing they can do about it over the phone is temporarily change my 15-yo's birthdate so he's 16 and then change it back after checkin. I declined this because it would probably kill the TSA-PRE status.
So it looks like I'll always have to check the kids in at the airport. Big drag in an airport that doesn't even have a real premier line (let alone GS).