How to tell a family that you're not the right babysitter for them?

I already asked a question about this situation but I think I want to tell the family that it's not working out or at least see what they think. The baby has really really bad separation anxiety and the entire time I'm there, she's crying unless her dad is holding her because he works from home. He ends up coming down pretty often to calm her down but we've tried everything including introducing her slowly but it's just not working out. I am scheduled to babysit again in about two weeks, so what do you think I should say to them? I've never had to do this so I'm not exactly sure how to word it or what to say. It's kind of awkward because they're friends of my mom. Also, I babysat two days in a row and there was no improvement whatsoever. I haven't even changed her diaper because the dad ends up doing it so I feel like I shouldn't even be there. Anyway, I'm just wondering exactly what I should say to them.
Answers

Laura

It is already scheduled, so its rude to back out even though you are giving them enough time to plan otherwise. I would let them know how you are feeling, that you don't think its fair to you or their child, and that if she is acting up again during your next visit you recommend finding another sitter.

Katherine W

You call them up now and say "I am terribly sorry, but I don't think I'm the right person to do this. I think you need someone different or more experienced. I wish you luck, but I won't be coming again in two weeks." That's it. This is their problem, not yours.

Tepee

You should be honest by telling them you feel frustrated because your methods of calming her have failed and you think they should find someone else who may have other skills that suit her needs better.