The Hiddenseek by Nate Cernosek

 Holly and Hector Thorn were playing a game of hide and seek and were never found, so they ended up in a alternate universe called "The Hiddenseek".  There they meet several other children who were also playing hide and seek and never found.  When IT finds you she turns you to stone.  As Holly, Hector and the others try to stay away from It, they begin to discover the history of how and when Hiddenseek was created and they try to fix it so they can get home, but things spiral out of control pretty quickly and it becomes a ticking clock of who will survive and make it out. 

Pretty good book.  Similar to Hide and Seeker, which I read and reviewed, but a little more historical feel to this one.