The ship was built for kids! Disney isn't stupid enough to face the legal liability of kids falling overboard.
The pool deck (9) had 5 or 6 foot high plexiglass walls which curve inward at the top. Unless you are Spiderman, you can't climb them. Deck 10 (where there is nothing for little kids!) has many high railings. Bottom line, the only way someone is going overboard is with a lot of intentional effort. TO the best of my knowlege, that has only happened once when a teenaged boy climbed overboard from the 4th (lifeboat) deck while the ship was in port. He was confined to his cabin for the rest of the cruise except when escorted by his father! And there's NO WAY that was accidental.