I've also traveled to Europe many times with children and discovered that most rooms will only sleep 2 people, usually in 2 twin beds. We've stayed in many bed & breakfast places and paid for 4, (2 adults & 2 children). They don't give you a break on the children, you just pay for 4 people.

I do recall one trip to London we stayed at the Marriott Marble Arch. We selected that property because they had 2 double (or queen?) beds and allowed 4 per room.

Recently we stayed at the Renaissance in Barcelona and each room had 2 twins. We booked 2 rooms for 4 of us. There was a charge for a 3rd person per room, but I don't know where they would have fit the rollaway bed.