There is certainly no rule that you can't sit in an exit row if you have a child on the flight. The problem is that you won't be able to prebook if if you are on the same PNR (as noted above). In fact, I have had a GA ask me if I would be willing to split up from my kids and sit in the exit row on 2 occasions. Of course, I said fine, but my kids were seasoned travellers and knew if there was any misbehavior, they would be dead meat.