With a Class C, you could do a 21 to 24ft. Cabover bed for the boys - they'll love it now at their ages, and by the time they're tired of it, they'll be on their own. Mom & Dad use the dinette bed. You may be able to find a bunkhouse model, but the bunk beds may not be as popular as the boys grow bigger, and unless Mom & Dad use the cabover bed, you'll waste space there that could be more useful. There are also some "double dinette" models that have the cabover bed, a dinette and kitchen midship, and a second dinette at the back - You could put the kids in the cabover, and use the back dinette as a dedicated bed for Mom & Dad.
As the owner of an "older" class C, I don't see anything wrong with buying older. But, make sure you:
1) do your homework - research, research, and research. Look up customer satisfaction of various makes. Study up on what to look for when you check out possible RVs. Be thorough, both on the chassis and the house.
2) be prepared to put in money after purchase to make necessary repairs at the outset. The more you pay for the RV, ideally the less you will need to put out at the beginning on repairs to get it in good working order (but that depends on how well you do #1). If you're mechanically handy, that will help defray costs.
3) do several practice runs locally. Drive it on day trips, etc. Get the motor running, so to speak, so you can catch any problems before that big trip.
1975 American Clipper RV with Dodge 360 (photo in profile)
1998 American Clipper Fold n Roll Folding Trailer
Both born in Morgan Hill, CA to Irv Perch (Daddy of the Aristocrat trailers)