Cube of Trevalia (Trevalian Magic, Book 1)

After her husband is killed when the cabin he's in burns to the ground, Kate Tucker must sell their house and move to her brother-in-law's farm with her two children.
Despite resistance from her teen daughter Susan, the move into the old farmhouse goes well. At least Kate and her brother-in-law Ethan both think so until Danny, Kate's youngest, takes an uncontrollable interest in the curious metal box discovered many years ago on the farm - the same metal box that was in the cabin with Seth.
The four of them converge on the box just as it comes to life. Finding themselves in an unknown world full of dangers, they begin the search for a way back home. But they aren't the only ones looking for the magical artifact that could provide that way home...