In the UK, hotel rooms don't tend to be set up for this, unless they are specifically sold as a family room or a triple/ quad. Most rooms will have either a double bed or two single beds. Even where the beds are larger, you should check if there are extra charges for occupancy above 2 people.
Travelodge certainly do a family room, with a double bed and two single beds. It's not available in every location, but if you put the search criteria, you'll find them.
That said, Travelodges aren't my first choice, and aren't classy!, but may be useful to you.
Example (not a recommendation, but because chosen at random because it has a picture of the room) in
Southwark
Hotels.com allows you to specify a room for 2 adults and 2 children, so that might be worth a look, too.