Oops, I missed this somehow.
Sorry.
The pop top upper bed is roughly the same size as the main double so I reckon as long as the kids aren't teenagers you'll be fine.
Yes the rear seating has 3 belts (only lap belts though).
as for luggage space, the whole rear area above the engine compartment should be big enough for all your stuff. There's also an external lash down point above the cab for stuff that is weatherproof.
