The Banishing Stones

Stella is ugly, flies a broomstick and can wield a magic wand. In these respects she is just like the other witch girls in Wartville. But, unlike the other witch girls, Stella does not enjoy turning toads into slime or setting the elves' forest home on fire. She would rather help a bear cub escape from its cage than cook it in a stew. It's only a matter of time before she finds herself in serious trouble.
Stella's fate is linked with the fate of two human children, Mitchell and Sarah, who don't even believe in witches. That is, they didn't believe in witches until they found themselves whisked out of their own world and into a completely different one, into the land of Wystovia. Mitchell and Sarah soon find themselves running from an evil wizard who thinks nothing of enslaving dwarves in the mountain mines, turning giants to stone, and terrorising innocent merchildren that his eagles catch frolicking in the sea.
Mitchell and Sarah will need a lot of luck and the help of friends to survive. But who can they trust? And is there any way to return home?