I am looking for a children’s book I read in 90s. I read it in the library of the language school.

A group of children find a gate that leads to another world, a world without any reflections (even when looked at surface of water). The gate itself is such that it does not pass any metals (coins burn red hot if attempted to be carried through). One of the children find out that their ancestors originate from this world, and that world is now under control by mean men. At the book first climax, a Locket Necklace worn by the child happened to contain small mirror, and it is used to distract and defeat the leader of the mean men.