I have to disagree.
Most every airline out there often changes things around, changing aircraft, consolidating flights, bumping people from the flights they booked to another flight at about the same time, etc..
Have seen this happen to me more than once. And, I have always booked way early, and selected appropriate seats.
In my personal experience, DELTA is the worst offender.
In fact, the earlier you book, the longer of a time window there is for one to be shuffled around.
I check my flights often, and have had to go in and change our seats because, yes, once again, the airline changed things around, and they do bother to try to keep your party together, at all.
ESPECIALLY since the OP of this thread was flying during the big post holiday snow-pocalyps (sp???) with hundreds of flights cancelled and changed around, I can see absolutely NO way to assume that this woman had found herself seated separately from her child due to her own negligance. In fact, I would go so far as to say that even if she HAD booked a better seat for her child, and assumed that somebody would be expected to trade seats with her, that this all happened exactly as she had planned.
Flying today is just a crap-shoot... russian roulette.