I'm looking for a chapter book that I read when I was ~9 in the early 80s. It's about two kids (boy and girl) who find a magic pencil that changes things to whatever they write. I think they get it from a handyman that uses it to change the length of his ladder, the type of cookies in a box etc. They change their size to 4" and have adventures. I remember a chase with a cat and them visiting a cave of some sort.