Yes you are right of course. You are all together anyway. When I booked my lot, I made a mistake the first time, and left 2 children to the dearer price ifykwim. But you cannot book children without an adult. So I had to book the children as an adult, fare-wise its all the one anyway, but unaccompanied minors cannot travel, so when they rocked up to check in, they would have had not record of an adult travellling with them. By linking the reservations, it showed they were not unaccompanied.
So, I guess the moral of the story is: Make sure you dont leave a child booking to Last!.