S.J.
You got great advise, I just wanted to add something to the one that mentioned the loud noise of the flushing. I did what most advised, but I also warned my sons that the toilet flushes really loud on the plane before we even got on the plane. When we got on the plane, I took them to the bathroom before it got crowded and had them stand at the door and went in and flushed the toilet. I did this so they knew what to expect if we had to use the toilet and so they won't get startled if we had to come back later and really use it. When I did this with my older one when he was almost 3, he decided he didn't want to use the bathroom the whole flight and was able to hold it. When I did this with the younger one, it didn't face him. He was fine with having to use the bathroom in the middle of the flight.