It is up to the counselors in the clubs, and he will have to be completely potty trained, no assistance is provided. The counselors aren't even allowed to go into the bathroom with a child. It also will depend on if the cruise is sold out or not. There are instances, where the cruise isn't sold out for adults but for children. They only allow so many of each age group onto the boat as to ensure that they will have admittance to the club. If there are a TON of 3 and 4 YO's on the ship, they probably won't admit your child, because there isn't enough counselors for an additional child. They must adhere to the guildlines of the counselor to child ratio.