Hello Amacasa and welcome to Cruise-Chat. Yes, it is always best to book six months or more in advance when you have special requirements, such as adjoining cabins. Since your child is toilet trained and so close to the age of 3, there should be a good chance of getting approved. The Oosterdam is kid friendly. All the cruise lines you've listed have children's programs. Some may have more activities than others though. Norwegian and Carnival are in the same category of Mass Market cruise line, and are a bit more geared to families. Celebrity, Holland America, and Princess are in the Premium category. Holland America certainly has programs though that will keep your children busy.