You people are so weird. If you can't afford prefered seating, a child under 13 should just be alone? How is that fair for the CHILD? Let's spend some more time judging people with tight budget and punishing their children for it.
Clearly, airline should book any child with a parent. If no two seats next to each other are available, they shouldnt let the booking go through. It's not rocket science.
I hate that mentally. If we lived in a world where the money you own was entirely based on merit and effort, I might get it. But we don't. You're lucky you can spare dollars for those upgrades. Other people can't or if they do, they'll have to cut somewhere else along the line.
Also, it is entirely true that airline DO NOT let you see all available seats.
The feature to pay to choose seat shouldn't even exist. If a family is split and you feel pressured into giving up a seat you "paid for", you should be mad at the airline who didnt block at least two seats together.
I can't always spare the extra for upgrades. I am never going to be willing to pay for a truly first class or business class seat even if I could afford it. So if I am booking a trip and trying to find a flight and find one that only has first class or business class seats left, guess what? I am not going on that flight. And that is my choice.
I do think there is a genuine issue sometimes with airlines moving people's pre-selected seats and breaking up families and I would agree 100% that is the airlines' issue. Or overselling a flight. However, this particular situation is not that. It appears that the OP purchased tickets at the lowest possible fare. The reason this is the lowest possible fare is BECAUSE it does not allow you to pre-select your seats. When you purchase these rates, you know and accept what the restrictions are. You are essentially allowing the airline to make YOU the person they move around arbitrarily if necessary because you are exchanging the right to choose a desirable seat for a desirable price instead. It is a choice.
It stinks that airlines now have all of these different fare categories but part of the reason they are able to offer the much lower fares with so many restrictions is because they are making money elsewhere via all of the upgraded fees and higher fares. What if they eliminated all of the upcharge options and just made every seat the same price? All of the prices would likely go up to cover the lost revenue from the upcharges. So now there may not be any fares at all that are affordable for some tighter budgets. Would that be a better option?
And again, let me say that if there are unsold seats on this flight, then I absolutely think the airline should do what they can to move things around to let the OP 's family sit together. I am not a heartless jerk who thinks a 3 year old should have to sit alone when there is an obvious solution. However, there is not always an obvious solution. Sometimes in these situations someone else is going to be inconvenienced to make something work for the OP. And while that is sometimes unavoidable and the fault of the airline (due to equipment changes, flight delays, overselling, etc), in this case that does not appear to be true.