Cube of Trevalia (Trevalian Magic, Book 1)
by L.J. Capehart
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.
The move goes well, until Danny, Kate's youngest, takes an uncontrollable interest in the curious metal box that was in the cabin with Seth when it burned down, and they all find themselves in a strange and dangerous world.