The Case of the Weeping Witch by E. W. Hildick, 1992.

This is one of the McGurk fantasy mystery books.

The kids are studying the history of their New England town in school. McGurk studies witchcraft trials, and he is outraged at how innocent people could be charged with witchcraft on little evidence and sentenced to death. When their teacher tells them that she has learned that a young girl was once put on trial for witchcraft in their town 300 years ago, McGurk convinces the others to try to use their “little black boxes” from the previous book in the series to go back in time and try to save the girl.

The girl, Hester Bidgood, is thirteen years old and turns out to be the goddaughter of Gwyneth, one of their friends from their last fantasy adventure. Gwyneth is now very old, but she remembers them and welcomes them into her home. A man in their community, Jacob Peabody, is trying to pressure Gwyneth into selling him her property. He is the one who brings charges of witchcraft against Hester. Hester calls herself an “investigatrix,” meaning that she is a detective, like the kids in the McGurk organization. She knows some disreputable things that Peabody has done, and Peabody wants to keep her quiet as well as force Gwyneth to give up her property. The McGurk organization is determined to save Hester, but they must be careful not to make anyone think that they might also be witches.

Of course, that turns out to be difficult because all of the modern-day members of the McGurk Organization have magical black boxes.

The book is currently available online through Internet Archive.

Spoiler:

Hester has a friend called Rob McGregor who was captured and raised by Indians when he was young. Although he now lives with his grandparents, he still has habits and skills that he learned from the Indians, and Hester likes to call him by his Indian name, Blazing Scalp (because of his red hair). I expected that Rob would turn out to be some ancestor of McGurk’s, just like the last book had a definite McGurk ancestor, because he has that distinctive red hair, but if there’s a connection, it isn’t definite.

Rob knows that Hester has learned that Peabody earned his fortune by cheating at cards and that he is hiding a mysterious guest in his house. Rob saves Hester when the people in the town try to conduct the water trial, seeing whether she will float or sink in water to determine whether or not she is a witch. Then, Rob and Hester hide in Rob’s secret lodge in the swamp while the McGurk organization tricks the man hiding in Peabody’s house into revealing himself.

The man, who has been going by the name Mica Holroyd, is really Matthew Hopkins, a former witch hunter who has been charged with witchcraft himself and is now a fugitive from the law. Once everyone in town realizes that Peabody is dishonest and harboring a fugitive, they know that the charges against Hester were false. Peabody and Hopkins are both sent back to England, never to return. When the McGurk organization returns to its own time, they learn that Rob and Hester eventually married and that the town gave Hester property that used to belong to Peabody as compensation for accusing her of witchcraft. Hester started a home for orphans and elderly people, which is now a retirement community in the kids’ town.

There is another book in the series which focuses on Rob and Hester alone, without the members of the McGurk Organization.