The exact meaning of magistrate varies widely depending upon the context. For example:
in the U.S. federal court: assists district court judges by handling minor offenses or administrative tasks such as preliminary hearings (often referred to as a magistrate judge rather than just a magistrate)
in some U.S. states: a judge in the state court
in France, Belgium, Italy, the Netherlands, and other civil law countries: a sitting magistrate is a judge and a standing magistrate is a prosecutor
in England: may be a volunteer without formal legal training who performs a judicial role with regard to minor matters
in ancient Rome: a powerful officer with both judicial and executive power
He told Dad that the whole family would need to come down to the courthouse the next morning and see the magistrate.
The magistrate would get to the bottom of the matter and decide what measures needed to be taken.
"That's up to the magistrate," he said.
There are no more uses of "magistrate" in The Glass Castle.