8 and 10, they are fine on the dinnette (a RV double, slightly smaller than a real double). My wife and I figure we spent the first 10 years of our marriage in a double every night, they can share a RV double for camping trips. Females in our families are usually in the low-5-foot range so length won't be a problem as they grow.
If they complain about the width when they get older I'll give them the option of one person on the floor with their feet under the dinette like a cross-wise bunk. I've seen larger travel trailers with a similar configuration. If I want to get really creative I could add 3/4 plywood above the dinette and make a bunkhouse out of the front of the 171, but that might have to be semi-permanent so we would loose the use of the table for a few years (then I would convert it back to normal). Not a huge deal since we usually eat outside and hate converting the dinette anyway.
If one of them decides that camping isn't cool when they hit the all-knowing teens then more room for the other!
