To those saying...rooms are tiny...it would be miserable etc... each to your own.
When we camp, we have 6 people in a space, 2m×5m (less actually, because my 6 year old ends up on my airbed).A hotel room is massive and luxurious. We could easily fit the whole family in the double rooms my DH and I share on occasion.
We have had holiday cottages for 6, where one bedroom wasn't used (and so was a waste of money for us) as one child slept in our bed and one on an air bed in our room.
We had to have 2 cabins on a ferry. There was loads of room for the 6 of us in the 4 berth cabin. Yes I followed the rules and slept with the youngest in the 2 berth cabin, but it would have been far better for us to all sleep in the 4 berth. We all used the same bathroom any way. The bunks were massive, plenty big enough for 2, and there was loads of room on the floor for an air bed.
I would never sneak an extra person into a hotel room, so we rarely use hotel rooms. But stop saying it would be miserable. It would be far more fun than having to split up, which is a complete hassle.
It is very useful information to know that some places would let us have more people that the room numbers if we ring up.
I can also recommend youth hostel rooms.