My daughters have always been good sleeper. I have heard some of my friends' kids never sleeping on the plane, but that was never been the case for my girls.
When my youngest was 22 months old and the oldest was 4, we went to Japan.
The oldest was already potty trained, and the youngest was still in diaper. So, whenever the oldest has to go to bathroom on the plane, we all went. When I had to change the youngest daughter's diaper or I had to go to the bathroom, I either left the oldest at her seat occupied with something or took with us to bathroom.
There was a time both of them were asleep and I had to go to the bathroom. I held it for a while which I am used to now a days after having kids, but could not wait any more. Noticed the both of the girls were asleep and the lavatory was vacant, I decided to leave them where they were and and just to to the lavatory 10 feet away without asking flight attendants for assistance. I thought, "It will be only a minute. They are good sleeper, so it should be fine."
One minute later when I came back refreshed, the youngest daughter who was asleep flat on two seat were GONE. The oldest was still asleep leaning against the back and arm rest, and had not move a bit. I began to panic a little, and moved a little forward on aisle to get closer to our seat in the dark.
Then, I found the youngest still asleep on the floor right in front of our seats. She must have fallen from the seats and still remained asleep. I knew there was no way she would be able to go missing in the plane.
Since then, even if they are both asleep, I decide to ask flight attendant to watch them while I go to lavatory. Even when I know I will be back in a minute.