Running Out Of Time by Margaret Peterson Haddix

In a gripping tale set in a seemingly 1840s village, a young girl named Jessie discovers that her entire life has been a lie, as her community is actually a modern-day tourist attraction. When a deadly diphtheria epidemic threatens the lives of the children in the village, Jessie's mother reveals the truth and sends her on a dangerous mission to the outside world to secure life-saving medicine. As Jessie navigates the unfamiliar and bewildering modern world, she must overcome numerous obstacles and evade those who wish to keep the village's secret hidden at all costs.

Purchase from Bookshop.org