I remember a movie faintly from my childhood during the late 80s-90s. These children go on a field trip to a castle like place and see a historial dungeon like room. Somehow they go back in time to a middle ages I think. This man dressed in black tries to capture them and he has these 3 white stones that he rubs together. At the end of the movie the children are back in London in the present and find their parents. I hope someone can help! Thanks!
