The Portals of Heaven and Hell — David White

Rick and Pam Jennings are desperately seeking a cure for their seven-year-old daughter who has Down syndrome. They accidentally find a way to travel back in time and make changes that will affect the present. They are hopeful that a cure for Down syndrome can be found in the past. But they soon find unexpected and bizarre consequences as they begin tampering with history. At one point they come close to accidentally killing their grandfather when he was a young man. While visiting him in the hospital Pam asks...What happens if he doesn't make it? Do we just disappear?
At last they find the one person who may have an answer and a cure. In the year 1870 they visit Doctor John Langdon Down, the man the illness is named for. But he tells them he knows of no cure for Idiots. It's not until they meet a Quaker missionary who lived among the Indians in 1775 that a cure is found. After Ashley is cured the entire family begins a series of adventures going farther and farther back in time. Each trip results in lives saved and lives lost. They learn that very small changes to the past will produce very large changes to the future...