Re: Is it possible...2 kids, 2 adults, 2 years in a SMB?
Go for it. You must be planning extended stays during your trip; otherwise, you can do this in less than 2 years. We just returned from a month long trip to the Everglades, and it was tight with 2 adults and 2 dogs. Not the same as kids, but they take up the same space on the couch in the 50 floor plan. We slept on top, which is the opposite of when we travelled with our 2 daughters in our '83 VW van.
We met a couple (from Tucson as well) at Hueco Tanks Historic State Park, Texas, with 2 boys younger than your children. They had a 2007 Chevy SMB (4x4?), built in Texas. They loved their SMB and used it for their rock climbing trips. He is a certified instructor. Their van was full of kid things. The car seats were the first things out of the van after the kids. I believe they had open shelving in the rear portion. They carried "crash pads" for climbing, which are very bulky, but everything fit into the van. No racks.
You will like the small size of the SMB during the driving portion of your trip.