Heaven Eyes by David Almond
Erin, January and Mouse live in a children's home, Whitegates. Running away is something they know all about - their dreams of escape frequently lead them into the outside world. But this time will be different. January has built a raft, and the trio head precariously down river, towards the Black Middens.
There they stumble across a disused factory and its strange inhabitants - ancient Grampa and a mysterious girl, Heaven Eyes. Heaven Eyes desperately hopes that the children are her lost family - but Grampa keeps the truth of her history concealed. While Erin befriends Heaven Eyes, dreaming of her own lost mother, and Mouse tries to belong, January's suspicions grow . . .