The Ritz Carlton has very large rooms for Tokyo standards and can be booked by transferring SPG points to Marriott -- may be worth looking at, especially if you were going to try to stay all in one room. Both it and the Westin will cost slightly fewer points when the new award chart kicks in sometime in August (the Ritz will go back up Jan 1). The Prince Gallery is also fantastic and centrally located, and will be the same pricing as the Ritz, but it has fairly small rooms so would definitely want 2 rooms for the family.

Japanese hotels are very strict about number of guests staying in a room and are typically more vigilant than hotels elsewhere, so if you're not planning to book 2 rooms already, make sure you're confirmed for 4 people in the room or you may have issues at check-in.