It's been awhile since I've been on one but some of the mainline "west metal" (sorry, I really hate to but I have to differentiate) 737's have 4 oxygen masks on one side and 3 on the other side. Otherwise, the 319's and 320's have 4 on each side, meaning a lap child could be anywhere.
Go with the exit row.
I agree with you, it is the parent who should make the child behave.