Murmel, Murmel, Murmel by Robert Munsch, 1982.

Robin is playing in her backyard sandbox when she hears a “Murmel, Murmel, Murmel” sound from a hole that she has never seen before. In the hole, Robin finds a baby. Since Robin herself is only five years old, she decides that she needs to find someone older to take care of the baby.

Robin asks various people, but they all have reasons why they can’t take the baby. Then, Robin encounters a truck driver who is enchanted with the baby’s “Murmel, Murmel, Murmel” and says that he wants him.

The story never explains where the baby came from, how he ended up in Robin’s sandbox, or if his parents are looking for him, but apparently, he’s happy with the truck driver. As for the truck driver’s truck, he says that Robin can keep it because he already has seventeen others. Robert Munsch books are like this. That’s basically the explanation.

The book is currently available online through Internet Archive.