I regularly book tickets for Miss and Master PVDProf, while checking the "adult" box, so I can book my ticket separately. Even when we've had irrops, there have been no problems--no mention of it--when rebooking, even before the SHARES process of splitting everyone's PNRs at check-in. They're not trying to stick you with a UM fee while you're sitting there; they're trying to make sure no kid ends up accidentally in their care without anyone knowing, or that they can demand the fee or deny travel when there are no adults traveling.