I remember almost nothing about it (helpful 😄) except that I think it was about a child (or children?) from our world who ended up in an alternative magical world. There was a nice/good witch whose name might have begun with M, and a wizard who I think started out as the villain but it turned out was actually a good guy and in love with the nice witch. The only part of the story I can really remember with any clarity was a scene where the wizard protected the witch and the child(ren) from something by enclosing them in a magical bubble? It would have been late 80s when I first read it and I absolutely loved it so would love to find out what it was!