I remember back when I was a child (late nineties), I listened to some audiotapes. One of them was story about two children, and a wizard who accidentally teleports into them when he is trying to pass his wizarding exams. They go off on all sorts of adventures, teleporting to different worlds, before realizing the wizard has grown in confidence and needs to go back to retake his test.
I listened to this tape in the late nineties, but the tapes (and therefore the story) might be older.
Does anyone recognise this story? Could you name it?