As for kids being split from their parents, the main reason this happens is due to overbooking by the airlines (which is legal, btw).
I have to disagree. Unless there is a connecting flight delay, the people who don't show up are most likely to have been business travelers who are on refundable coach fares. Those are NOT the people who are likely to have reserved middle seats.
I believe that the single-seats only phenomenon results primarily from schedule and equipment changes, and also from pricing systems which are keyed to yield mgmt software. When a flight time or type of aircraft is changed, even by so much as a few minutes, the seats will be reassigned by a computer, not a human being. That computer reassigns seats based on FF status and the fare class of your ticket; the higher both are, the better the seats the system will give you. Also, in the case of no-shows and last-minute cancellations, the best of those seats are almost always offered to the highest-status or highest-fare coach passengers as an upgrade, so the leisure traveller who is waiting for seats gets bumped down into any undesirable seats they upgrade out of.
For the past couple of years on most legacy carriers, the most heavily discounted leisure fares do not allow advance seat assignments at all. In my experience this appears to be because those fares are actually *priced* as middles-only, and the only way you will get into a window or aisle seat if you have one of those fares is by benefitting from a no-show situation.
Oh, and for the person who feels it it is wrong for young children not to be seated next to parents, my best advice is to write your Congressional Rep. to complain. FAA regs require the accompanying adult to be seated next to the child only if the child is in a carseat. If no carseat is being used, the only restriction on the seating of children is that they cannot sit in an exit row. Children may legally fly unaccompanied at age 5, and the legal definition of "accompanied" has been interpreted as "on the same aircraft." The only way that airlines are going to force passengers to move for this reason will be if the FAA regulations are changed to require them to do so.
PS: I should have mentioned that IME, the airline most likely to try hardest to coerce passengers to move to accomodate parents/children is SWA. They will ofter free drinks for trading quite often, and if the child is pre-school age, they may even threaten that the plane won't take off unless someone trades.