This is where the dual requirement comes in to play, and some of us have seen it enforced at 7DMT. It's not just that you are tall enough, you also have to be old enough to ride without someone 14 or older.
Again, many have gotten by with no issue. If I had only seen it once I probably wouldn't have thought twice but I've seen it many times (and we ride a few times every trip since the day after it opened) including the one huge scene that left that family standing there. No telling what will happen in each case, as it probably depends on CMs at the time.
But also to note that they are to fill every seat. When you enter final room you either go in the odd or the even line. This way they put odd groups together, so one child will probably ride with a stranger. Sometimes by the end of the car they are out of matching groups and time (got to keep it rolling) so a few trains may get off with empty seats. It would be good for your child to know in advance they might ride in with someone else.