I cruised when my kids were 2 and 4 and again when they were 3 and 4. Also when my oldest was 21 months old and I was five months pregnant!
There is a curtain the separates the adult and kids sleeping areas. Just a FYI, the cribs are pack and plays and I used them when the kids were 20 and 21 months. However, it was not very comfortable (plywood boards covered with very thin padding. If you DS is old enough and you feel he can handle a bed, the bottom bunk is only a foot off the ground and they can give you a rail. Problem is that top bunk is really for 6+ (even though DD used it when she was 4 almost 5 - It was her most favorite part of the cruise), so they will put in a pnp. You, however, can choose your sleeping arrangements.
DH and I pushed bedtimes slightly, knowing that we could sleep in (meaning to 7 am!) and took turns watching shows, since the room had to be dark, no TV, etc. One adult got to have fun (shows, movies, walking outside) and the other adult was stuck in the room at 8! Honestly, I was so tired that when it was my turn in, I was asleep by 9! Even staying in a veranda room, the door is very heavy and I thought loud (when you have barely sleeping kids) so it wasn't really an option at night. As an aside, I had a hard time opening the veranda door. And there is a latch that is high up.
Some other things to know....
1.) Unless your DS (and DD) are fully potty trained they cannot go to the kids club during secured programing. Fully Potty trained means clothing management (up and down), wipe and wash hands. Anyone can go into the clubs during open hours!
2.) Unless they are fully potty trained they are not allowed in the pools. No swim diapers allowed. There is a splash area for the youngsters who are not potty trained (your oldest would be okay for it as well, even if she IS potty trained!)
3.) Early sitting is at 5:45 and can take up to 7:30/7:45. You can ask the servers to give the kids their main meal while you have your soup/salad/appetizer courses. We would then take them to the nursery/kids club and enjoy the entree and dessert/coffee alone!
4.) Kids club (3 -12) is drop off. and Nursery requires reservations. We reserved from 6:30 - 8 every night, extended the hours if we were going to adult dining.
5.) If you go to adult dining (Remy or Palo) then make sure you feed the kids first before sending them to Club/Nursery since they only have cheese sandwiches and don't enforce them eating.
6.) Strollers....They halls are very narrow as well as the stateroom doors. I have a double side by side travel stroller that we use that is the same width of a wheelchair and I had to fold it up to get in stateroom. Also the halls have service carts, other people and can get congested. I found that on the actual ship we really didn't need the stroller, but we used if for the airport and port excursions. Elevators can be very tricky with a stroller and we often had to use the trick where we would "call" an elevator going in to opposite direction as the masses and then come back to the same floor and then do our destination, since it was the only way we could get on the elevator.
While you are less things for your youngest to do, he will just enjoy being on the ship! We didn't do any excursions, just would find a beach that was a short taxi ride or walk able. And off course, who could beat having Micky and Minnie be with you on vacation!