Bingowings said:

Why don't you go upstairs under the pretense of having a headache and asking to borrow some asprins and casually ask if everything is okay while you are there?

If your neighbour asks why, explain that your headache has just made you more sensitive to sound and you could help notice the crying child but you understand this is the sort of thing children do.

If you play it right you will get the sort of reassurance you want without looking like a nosy busy body.

It is actually next door, and not upstairs, and in my time zone it was 1:30 in the morning, well past the appropriate time to go knocking on the neighbor's door. I went out and had a smoke, then came back in. The crying had stopped, but I still felt unsettled and had trouble sleeping for a while. It seems my bedroom shares a wall with a small child's bedroom.

I am now extremely bothered by this. I had a particularly taxing day at work yesterday and went straight home to go to bed, rather than going out like I usually do. Sometimes my Saturday nights involve having a female friend over for the night. The realization that there is a little kid sleeping just on the other side of the wall, the very wall my headboard is pushed up against, is really bothering me now. I can't stop thinking about how some of those noises may be making their way through the wall and are being heard by a little kid...