Trapped in Time by Ruth Chew, 1986.
Audrey (called Andy) and her younger brother Nathan are having a picnic in Prospect Park in Brooklyn, New York, when Nathan falls and knocks over an old tree stump. They spot something shiny in the deep hole under the old stump, and Nathan climbs down to get it. It turns out to be an old pocket watch, the kind that needs to be wound with a key. Nathan has a small key in his pocket, made for a toy, and decides to try it on the watch. He manages to wind it, but suddenly, the hands of the watch move backwards, the children feel strange, and everything around them seems different. Although it takes the children a little while to realize it, they’ve traveled back in time about 200 years, back to the Revolutionary War.
They meet a boy named Franz and become friends with him. Franz is a drummer boy for the Hessian soldiers, Germans hired by the British to help them fight against the rebelling colonists. Franz’s superior orders his men to commandeer food and supplies from the people living in the area. Franz is supposed to take a family’s cow, but the family desperately beg him not to because they have nothing left and will need the cow to support themselves. Andy persuades Franz to leave the family and their cow alone. However, disobeying the order means that Franz will be in trouble with his superior. He decides that the only thing to do is to desert the army.
Franz had only joined the army in the first place because his parents were dead, and he didn’t know what else to do. Now, he has to find a new place to live, somewhere where there won’t be other Hessians who would recognize him as a deserter. Andy and Nathan also have problems because they’ve now realized what time they’re in, and they don’t know how to get home. The watch no longer seems to work.
The children travel together, meeting others who help them and seeing the effects that all of the armies, British, Hessian, and American, have on the ordinary people as the war continues around them.
When they finally find a place where Franz can stay safely and someone he can call family, who can also use Franz’s help, Andy and Nathan realize who the watch really belongs to and how they can return to their own time.
The watch’s real owner is the person I thought it would be, and it took the kids unexpectedly long to realize it. There is a hint of what happened to Franz when the kids finally return to their own time, but I kind of wished that they learned more about what Franz’s life turned out to be like. From what the kids see, it seems that things went well for Franz and that he continued living in the house where they left him.