All The King's Horses — Laura C. Stevenson

'REMEMBER ALL THOSE STORIES GRANDPA USED TO TELL US ABOUT CHANGELINGS...?'
Colin and Sarah can't bear the way their much-loved Grandpa seems to be slipping slowly away from them in his old age. Refusing to believe it, they decide instead that he has been stolen away and a changeling left in his place. In an attempt to find him again, they follow his path, step by step out of the land of mortals and into the Otherworld - the realm of the Faer Folk...