Book 2 rooms only if you want 2 rooms, OR the room capacity will not allow for 2 adults AND 2 children. (I assume 2 adults with 2 children).

If you want to stay in one room, and you can book the bed type you want and the hotel will allow 2 adults and 2 children, then you should be fine with 1 room.

I have often booked 3 in a room and have never had to pay any additional breakfast/lounge charges.