Witness (1985) on Netflix

Young Amish widow Rachel Lapp is taking her young son Samuel to Philadelphia for the first time. While in a washroom at the train station, Samuel sees two men savagely murder a third. The detective assigned to the homicide case is Det. John Book who informs them that the murdered man was a police officer. It isn't long before the young boy fingers Philadelphia narcotics officer McFee as the murderer. Book soon discovers that McFee was involved in the theft of something called P2P, an ingredient used in the making of methamphetamine. Shortly after Book tells his boss this information, McFee comes after Book and wounds him in a shootout. Book determines that the best way to avoid detection is to blend in with the Amish community, which he does, working with them on their farms and helping with building a new barn. But the corrupt cops Book has exposed are determined to keep their dirty work a secret, and they come looking for Book & Samuel to silence them for good....