The Hiddenseek by Nate Cernosek
After Holly and Hector Thorn are left behind during a simple game of hide-and-seek, they find themselves transported to a gloomy, magical world seemingly trapped in time - the Hiddenseek. There, It, a witch with the ability to transform into a raven and a wolf, stalks children day and night, turning any she can lay hands on into statues, frozen forever in their final moments of terror. Together with the other lost children they encounter, Holly and Hector will have to unravel the mysterious origins of the Hiddenseek and find a way to put a stop to the curse once and for all, before they get stuck there forever... Because in this twisted game of hide-and-seek, they are playing for their lives.