If I was going to take a vacation with my grand kids I can honestly say the cruise idea has merit. It is all inclusive in what that means, some food will always be included. You can get special food or add alcoholic drinks but I do believe there is a fee for those higher priced items. You'd need to check for sure as to the exact things that are supplied.
The hotel room is included, food, entertainment, babysitting, all sorts of activities, clubs, shows, movies, ice skating on some ships, Disney Musicals or new movies on others....so much to do.
The prices can be overwhelming for sure. But when you realize what you are getting for the cost it is very cost effective. You don't need a huge expensive room, it will mainly be just for sleeping. The further inside the less motion too. The cruise line will need to be considered for sure. I want to ice skate on the ocean so I would chose Majesty of the Seas or one of her sister liners that have that option. The kids are going to be super busy in the kids clubs so mom and dad will have lots of time to themselves for sure.
So all in all a cruise is really a great deal. If you Depart from Boston you can choose many that do the New England coast. They are especially popular in the fall.
Here's one from Expedia.
It starts out in Boston and visits some Canadian cities then brings you back to Boston.
http://cruise.expedia.com/Itinerary.aspx?item=429818
Now if you could leave from another port then you'd have the whole world open up to you.
I want to sail out of Port Canaveral when we go. I want to have extra days to sight see there. I have a list of ports I want to visit too. I have no interest in Nassau. I don't want to visit the straw market or wander around, I want to have destinations to see and things to do. I want to go to Freeport more. It's the capitol and houses their Parliament, there are also about a dozen forts from the pirate days there too. Much more interesting to me than the other island.
So a cruise is a good alternative.