Escape from Mercy Hall (Thorn Gate Trilogy)

In trouble again with the unpopular headmaster of Mercy Hall, Jethro Barking, Milly escapes from the orphanage with the help of her friends George and Samuel, and sets out to find her lost older brother, Tom. Little did any of them expect to be bundled through a thorn hedge by shadowy figures, into a strange land of Muttons (flesh-eating sheep), and forbidden Changelings. Finding Drago, a slave dog, staked out and at the mercy of the Muttons, they save him. In doing so they learn about the mysterious Rainbow Cave and the strange people, called the Robes, who control the cave and the glowing city beyond. Aided by Drago, and Batty the Tick-Tock bird, the children travel through the Rainbow Cave itself and discover the Dome, an enormous electro-magnetic power generator, worked by slaves kidnapped from their own world. Can the children escape? Can they help the slaves escape? And are they now changelings themselves?