I have done several of these cruises and have cruised with kids as young as 2. It kind of depends on the child as to how they are on board. The 2 year old did not get a nap and was not having his best night, yet another cruise with a boy the same age was a dream!
As for the bathroom, this is a tough one. On my first cruise I had eaten something that did not agree with my stomach and let me tell you, it was one long hour. There is no place to even disembark the boat unless you to back to the dock and once Illuminations begins this is not an option.
LInda