As a fourth grader in 1980, I had a teacher who kept a personal library of paperback books for us children to read when we were finished with all our other work. I read dozens of books in Mrs. Cohn’s classroom.
So assume this book to be anywhere from 1950s-1970s publication, a novel but fairly short. Things I recall:
There were at least two boys on the trip to another world/realm/dimension which they may have accessed via a cave.
Somehow they were equipped with a small wooden box of sausage and cheese. Intending to conserve their rations, they only ate half of each and discovered upon reopening the box that the food had regenerated to the full, original proportion.
They also had a blanket that would cover them both but fold down to pocket handkerchief sized and a “magic” match that could be struck and used again and again and even be stuck in a crevice and (perhaps rotated?) to become bright as a torch.
In one scene they passed a gangrel/beggar/wastrel on the road and as he came toward them he morphed into a well-clad, upright gentleman with a sandwich board or a handbell and advertised some type of ware or service and upon passing, returned to his former low state.