Your children should stay together. Our last cruise my DS was a month from turning the age to be in the same group as our DD. I requested for him to be in the same group. The CM's let him in on a trial. I knew up front if he could not be as mature as the others he would be out and into the correct age group the next day. If you are asking why it was a big deal to us, it was not because we thought they needed to be together. It was because this way they would be offered the same activities at the same time. Thus, if they wanted to do a specific activity I would only have to worry about getting them there once. This just made it much easier for us.