Just got back from Tokyo yesterday and have stayed at Andaz, GH and HR over the past two years. I have two daughters ages 12 and 11.

If your goal to have as big a room possible, then the Hyatt Regency is the best. I have opted for HR, even though I don't find the service to be all that great by Japanese standards. It's the best option to get to department store basement groceries for ready made food for the kids. It's a good half mile walk to the closest department stores but you can be under a covered walkway with a moving escalator for much of it.

If your goal is to have good food in authentic Japanese restaurants that can accommodate kids and where you can hope to have an English menu I would go with the Grand Hyatt, as you are in the Roppongi Hills complex. The restaurant options near the HR are limited in comparison.