If you're interested in European style class B campers you might consider the Hymer Aktiv 2.0 Loft Edition:
Hymer advertises the Aktiv Loft Edition for families up to six, but that's assuming two very small children sleeping crosswise on the bench seat. A more realistic arrangement would be two full sized sleepers on the main rear bed and two more in the pop-top loft.
Note this type of Euro-style camper does NOT have rooftop air-conditioning. So it's more like a classic VW campervan on steroids, plus luxury touches such as bathroom/shower, microwave, TV, etc.
When the twins are grown you may find you prefer sleeping up in the loft anyway. Although we love our current Sprinter van the most memorable experiences were from our old Westfalia pop-top, such gazing out over Arches National Park and the La Sal mountains perched in our own private treehouse. It's a unique experience!