The idea that any airline would try to seat a young child away from his or her parent, whether allowed by law or not, is absurd to me.
I mean, if you want to seat my kid halfway up the plane, go for it, I guess, and we'll see how that works out? I'll have a nice relaxing flight while the lady sitting in row 11 now has to babysit my 6-year-old, who is likely shrieking in distress over being surrounded by strangers. I'm thinking you're going to have a lot more unhappy passengers than just me and my kid.
I do not pay for advance seat selection, on principle. It's an unnecessary cash grab. I don't expect that my family of four will always be seated together, but I do expect that my young children will be seated with a parent. I have never had any problem at all checking in at the 24h mark on WestJet, even on fully booked flights. I have *twice* had issues with Air Canada pre-allocating my family into an emergency exit row, and being unable to change it at the 24h mark because every seat on the plane has been pre-allocated (and I don't believe for a second that every other passenger on that plane either paid for seat selection or got in ahead of me). In both cases, the (grumpy) gate agent had to deal with it. We now avoid flying with Air Canada whenever there is a reasonably priced alternative.