Maybe a diesel, full size crew cab truck would work. It probably has close to the interior space of the MB GL350.
You could remove the rear seat of the truck and install a carpeted platform for the 'kids.'
Another option, depending on your budget, would be a custom built six door truck.
Custom Autos By Tim will not be converting 2017 Ford trucks into Excurcions.
But, they still build six door trucks.
Probably a used diesel Excursion would be the best choice. You may want to stay away from the 6.0 diesel. Many have had problems with that engine.