The House in the Cerulean Sea by TJ Klune (Cerulean Chronicles #1)

Linus Baker leads a quiet life. At forty, he has a tiny house with a devious cat and his beloved records for company. And at the Department in Charge of Magical Youth, he's spent many dull years monitoring their orphanages. Then, one day, everything changes. Linus is summoned by Extremely Upper Management and given a highly classified assignment. He must travel to an orphanage where six dangerous children reside - including the Antichrist. At the orphanage, Linus must somehow determine if these children could bring about the end of days. But their guardian, the charming and enigmatic Arthur Parnassus, will do anything to protect his wards. And, as Arthur and Linus grow ever closer, Linus must choose: his duty, or his dreams . . .