The Borrowers by Mary Norton:

Beneath the kitchen floor is the world of the Borrowers — Pod and Homily Clock and their daughter, Arrietty.

In their tiny home, matchboxes double as roomy dressers and postage stamps hang on the walls like paintings.

Whatever the Clocks need they simply “borrow” from the “human beans” who live above them.

It’s a comfortable life, but boring if you’re a kid.

Only Pod is allowed to venture into the house above, because the danger of being seen by a human is too great.

Borrowers who are seen by humans are never seen again. Yet Arrietty won’t listen.

There is a human boy up there, and Arrietty is desperate for a friend.