You do realize it's possible to do laundry at least once during a 6 week trip? So you don't need to bring 42 outfits?

Thank you all for your help. I think we'll stick with the mini van and use duffel bags for the kids.

Duffel bags are a great idea (honesly, for everyone). We recently bought completely soft-sided (read: crushable) Eagle Creek bags that open like a suitcase but also have backpack and regular straps, and handles, no wheels so they are light as a feather. They are so easy to handle, and can be stuffed into much smaller spaces than a regular suitcase (like an airplane overhead - they're so light I have no trouble lifting them over my head either)

I'd recommend a collapsible cooler bag also..easier to pack than a hardsided one (although doesn't keep food cold for quite as long). May a small igloo too keep a few things in the back seat, in addition to a soft one in the trunk space.

An even better style duffel we have used is one with a compartment in the bottom. We were on a European trip and needed "dressier" clothes for a couple of occasions and could leave these undisturbed in the bottom.
I am still wondering how long this trip is and why the adults need "larger than carryons". Coolers are a good idea. The kids will want a backpack something to keep their media needs together and organized.

They said 6 weeks.

Why did I think it was 6 weeks?