Wow - some of you have some serious animosity towards children.
I can assure you that my perfect little angels have no interest in being in the adult area, nor would I allow it. Frankly, I have no interest in being in the adult area and have never taken part in this on any ship, even when cruising without children.
That being said, we can't be responsible for the design of a ship which mandates that one has to walk-through the adult area on the way to certain parts of the ship.
This was the design of Disney Dream, which has you walk through Serenity Cove to to get to parts of the ship. I'm not going out of my way to go up/down a deck to get where I need to go.
Further, I think once a child is potty-trained they generally have pretty good control of their bladder. I know for a fact my kids have never ever ever peed in a pool. I'd personally be much more concerned about the adults with the incontinence/mobility issues.
Finally, as someone above noted, its not always easy to tell a person's age. On Disney Dream, my tall four year old got kicked out of the splash pad area and was told he was too old. When I looked at the posted age limit, the area was for those 8 and under. Old or young, its not always easy to tell someone's age.