It's listed The Oddmire book 1, but it's a standalone story.
It briefly recounts how the magical world and the human worlds got separated, and the wall between them broken, and how a Thing got out.
Then to the main story. The goblin Kull brings a changeling, despite orders, so he can take a human baby. Then, having put the changeling in the cradle, he's prevented from snatching the human baby, and the changeling changed so well that he can't tell which one is which.
Neither can the humans, down to the baby's mother. So, they decide the goblin won't be able to hide its nature too long, and wait. Except that boys are mischievous too. . . .
When the twins are about to turn thirteen, Kull writes them a letter -- he can't speak to them by law -- and tells them the changeling has to come to the forest, or both the changeling and all the other goblins will die. No human must come. So, of course, Cole and Tinn go, leaving a note for their mother. . . .
It involves the last tale of when goblins sent a changeling, a witch in the woods, a hinkypunk with a candle, raspberry tarts, the mother's reaction to that note, bones, a map that Tinn mislays, and more.