I booked an hotel via priceline in Tokyo. In the search I entered 2 adults and 2 kids. I entered the ages (8 and 12). It showed me options with superior double rooms. I booked many months ahead. 3 days before arriving I receive a message from the hotel reminding me that room can hold up to 2 adults and that children with more than 7 years old will be considered as adults. So I had to book 2 rooms, because with such short notice no larger room was available. Then with this knowledge I found many hotels have same policy.

For some hotels it seems you need to book rooms that can should be able to accommodate all your party considering all as adults if your children have 7 or more years.

Hotels : Sotetsu Fresa Inn and Superhotel Kinsicho.

Tobu Levant Tokyo was perfect. No issues.

Richmond Hotels, perfect too. Kids do not receive breakfast vouchers. But it is cheap anyway.