Sara! This is one of those Duh! hacks I wouldn't have thought of myself, either!
If you are giving up a guest room in favor of two kid's bedrooms, do like my friend did: Put a full-sized bed in one bedroom and bunk beds in the other. Then, when people visit you can double up the kids in the bunk bed room and give the full-sized bedroom to your guests. (Obvious to some, but a revelation to me!)
I'm a bit nostalgic for the practice of bunking the kids (in bunk beds, sleeping bags, whatever) to accommodate guests. To me, it's a wonderful, concrete way to teach kids about hosting guests. On the other hand, not every guest feels comfortable sleeping on Thomas the Tank Engine sheets surrounded by stuffed animals. What do you think?