Chosen, I couldn't agree with you more.
And Phoenix, think about your boundaries beforehand- what you are willing to accept and what you're not willing to accept.
I took a job with a lady "Jane" (I posted about her before) and loved her and her family. The first month or so was great- Jane and I became really close and were almost like best friends. As great as that may sound, it ruined both our business and personal relationship. She started REALLY crossing the line of treating me like a friend and not the nanny/ employee by not paying me, by overworking me, and by talking about some really inappropriate things (ie. her sex life, marital problems, etc). Although I DO talk about personal things with my friends, it was a little too over-the-top to talk about that stuff with my "boss". Because she saw me more as a friend than the nanny, she just took advantage of me. I had several serious talks with her about it and although she promised to change, she never did and I no longer work for her.
I would STRONGLY recommend thinking about these things before accepting a job. Like Chosen said, you shouldn't feel you have to cancel previous plans you had just because the parents called you for some last-minute childcare.
Good luck!