The House in the Cerulean Sea by T. J. Klune

Linus Baker is a case worker at the Department in Charge Of Magical Youth (DICOMY). He oversees orphanages with magical beings to ensure they’re up to code. Until one day he’s summoned by Extremely Upper Management for a highly classified assignment, visit a top-secret orphanage for a month and write detailed reports. He takes his cat and takes the train until the end of the line and opens his documents, only to discover that, among other children, the literal six year old Antichrist is on the island.

In the beginning it felt like many other fiction books, but the Antichrist angle—and other relevations that would be spoilers—made it interesting. I liked this book.