It's a mystery. And the booking logic does not correspond to the safety justification. If the answer is that you need to have an adult across the aisle from a child in order to help them (which makes sense), they should code that into the booking engine. But that's not what they did. For example I am able to create a booking with an adult and a child where they are both in window seats a row apart. But it won't let me choose a window and then an aisle in a different row. So clearly there's a bit more logic going on than just "child in window"...and I'm not sure why they won't for example, allow the coding situation where there are multiple adults with one or two kids...