I believe the OKW stuido with 2 Q sleepers (and room for a air matress for your son-you would have to bring the matress/linens). But since you would be within occupancy it is fine.
Or depending on your son's age BWV has a "daybed" which also could be used for a younger child, giving mum a fullize sleeper that is very comfortable to use. BUT you are not gaurenteed this type of room and not all rooms have the "day bed"
Ok I looked at your profile...you son will be 10 at the time of your trip? You might choose one of 2 things...Getting a 2 brdm or consider sleeping in a stuido at OKW with removing the top sheet and folding it in 1/2 length wise, then turning it on it's side so that it makes a C(get a second top sheet from house keeping. This way your son and mum each have their own kind of sleeping bag. I think the 2 queens will give you the most room in a stuido because the sofa sleepers are fullsize/double beds