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.