House in the Cerulean Sea by TJ Klune

Linus Baker is a by-the-book case worker in the Department in Charge of Magical Youth. He's tasked with determining whether six dangerous magical children are likely to bring about the end of the world.

Arthur Parnassus is the master of the orphanage. He would do anything to keep the children safe, even if it means the world will burn. And his secrets will come to light.