Clues in the Woods by Peggy Parish, 1968.
While Liza, Bill, and Jed are staying with their grandparents, their grandmother starts saving table scraps to give to some neighboring children who have adopted some kittens but who don’t have much money to buy food for them. Unfortunately, someone keeps taking the scraps when the grandmother leaves them by the back door. The grandmother even tries leaving them inside the can by the back door to make sure that no other animals can get to them, but they still disappear. Although it doesn’t sound as exciting to the children as solving their family’s old treasure hunt was, the Roberts children decide to take on the case for the sake of the poor kittens!
The kids try sitting up and night and watching from a window, but Bill’s impatience and temper keep them from seeing what happens at the right moment and causes them to accidentally break their grandmother’s lamp.
They adopt a puppy from a neighbor and keep him on the back porch, hoping that he’ll bark if anyone trespasses out back, but the puppy suddenly disappears! Can the children find both the scraps thief and their poor puppy, Jelly Bean?
Just when the kids think that they have everything figured out, they learn something surprising that changes everything!
This is part of the Liza, Bill, and Jed Mysteries series.