I agree with the sleeping bag idea, and kids love them.
If you look to do a lot of traveling, I would buy something meant for camping, rather than the occasional kid sleepover, as the cutesy ones with characters on them don't hold up well. LlBean sells nice ones.
You can lay a waterproof pad underneath them, and everything washes up nicely. The bedding then stows easily, too, minus all the conventional bed trappings.
We decided years ago that unconventional beds that must be made up every night call for unconventional bedding, and it really simplified our travel.
We went to an all season down comforter, with a washable duvet cover over it, and a quilted mattress pad. Voila!
Custom sheets are ridiculously expensive, in my opinion.
We found several years ago sleeping bag
liners somewhere...maybe Cabela's, or some other camping outfitter. They were on clearance, $15 or so, and we bought several.
They are a sturdy, heavy fleece, with equally sturdy zippers, and my grands love them because they are so cozy. When it is cold, they are covered with something else.
Once you let go of the bed image in the promotional photos, it gets much simpler
Have fun!
Maggie