Drop down? Not aware of any in a B. But pop UP? Yep, those exist. Check out Sportsmobile, GTRV, Colorado Camper Van, and a host of others.
If you're willing to go up to a B+/C size rig on the Sprinter chassis, there is a unique new "mini A" in the form of the Winnebago Via and it does have the option of a drop down studio loft over the cab area.
Beyond those, one of the best options I've seen for a class B is an aftermarket unit from Reimo that converts the front seats into a bed (Link Here
There are also lots of DIY solutions for creating a loft/bunk out of PVC and cloth and suspending it from the ceiling via the front doors (Link Here
). Looks like it would work great for kids.