Running out of time - Margaret Peterson Haddix

'I may have to ask you to do something very dangerous,' Ma said.The children of Clifton are dying and Jessie's mother is desperate for medicine to save their lives, but it is 1840 and no such remedy yet exists. Except that it isn't 1840 at all, it is 1996, and the critical situation forces Jessie's mother to reveal an unbelievable secret. The world Jessie has known all this time is just a sham, a living history museum, a reconstructed village for tourists. And now her mother wants her to escape into the twentieth century to get help - before the children run out of time.