you know I can't add to much to what has been said. Those of us who sail NCL a lot are thrilled the Sky will be doing short cruises, Many years ago they did them, then got out of the market. I have sailed the Sun, it is true the cabins are a bit small, but the decor is light and bright, giving the opinion of larger cabins.
Your son should have a great time. They do allow 2 year olds to take part in the kids programs but if he isn't potty trained you will need to be available to do the changing bit, that is one thing they will not do.
Actually, there is some truth to the remarks about "freestyle" not always free, but it is a great program when traveling with little ones. Let's face it, no matter how good a 2 year is and how flexible, they do have minds of their own. Being confined to eating at a certain time can be challenging. With freestyle if he is tired and ready to eat at 5:30 of 6 you will be able to keep him happy. If everything is going well, he is having a good time, you may want to eat a little later.
I think you will enjoy your experience. Do remember one thing, if you have never done a short cruise they are not quite as elegant and the service and food is not always as good as 7 day and longer cruises.
Nita