At one point during the fight scene between Tony and Truck, Dermott Downs punched the bat so hard, it hit Ike Eisenmann. In some scenes, despite the makeup department's best efforts, you can see a greenish bruise under his left eye.
Kim Richards and Ike Eisenmann, who play the children in this film and in its 1978 sequel Return from Witch Mountain (1978), appear together in a scene in the 2009 remake Race to Witch Mountain (2009). Eisenmann is "Sheriff Antony" in the bar, and Richards is "Tina" the waitress. In this 1975 movie and its 1978 sequel, their names were similar: "Tony" and "Tia", respectively.
The geographical structure which portrayed "Witch Mountain" was a Santa Lucia Mountain in the Santa Lucia Range in Big Sur, Monterey County on the central coast of California. Candidates for a real-life Witch Mountain would be mysterious places like Silbury Hill or Mount Shasta.
The make and model of the chopper which was seen flying upside down and landed that way was a red-and-white Hughes 369HS helicopter with the branding of "Aristotle Bolt", which was the name Ray Milland's character in the film.
The name of the mysterious item that Tia carried around, which contained a map, was a "star case". Tia's star case, which houses the map to Witch Mountain, had an additional purpose in the novel. In Key's novel, Tia was mute (but communicated telepathically with Tony) and carried pencils and paper in the star case to write notes to people.