I'm a teacher, so I get asked to write recommendations for students all the time. This is my advice:
Unless you really didn't like her or thought she was a terrible teacher, write the recommendation. Telling her that you can't write it will suggest to her that you thought that she was awful. I almost never refuse to write a rec for a student.
Find a way to highlight the things that she is really GREAT at (loving person, good with the kids, etc). It will then be clear that she's only average at teaching. But she is great at some things. You then are writing a positive and honest recommendation. Anyone worth their salt will be able to read between the lines and see this person's strengths and weaknesses.
Good luck.