Help! Advice needed!

No need to feel guilty. You do not need to explain why you can't work. A polite sorry, I'm not able should suffice. If they needed someone full time, they should have advertised for full time. Just my opinion.

I had this problem with an employer once. I think if you are upfront, especially emphasizing the fact that you have to have prearranged child care in place for your young child, they will be more likely to understand. I told my employer that the reason that I worked every weekend was because I needed to be home to care for my young child, as my husband's job schedule was unpredictable and he could have to leave or stay away overnight at any given time, except on the weekend which was when I was available to work. My supervisor said to me. "I know and I understand. But because of policy I have to at least call and ask/leave a message asking if you could come in or I could get in trouble with my supervisor for 'not trying'". I felt better knowing that she was only calling me because she had to and not because she expected me to figure something out and pull a sitter out of thin air in order to come in when I was unavailable.

I agree, no need to feel guilty. You took a part time job, not a full time one. If they need someone full time they should've hired someone for full time. It is not your responsibility to work over the amount of hours you are hired for. You don't need to give any explanation other than no.

I agree with the other posters that you shouldn't feel guilty if you're unavailable to pick up extra shifts. I also wanted to add that your supervisor has a responsibility to exhaust all options when trying to cover open shifts. I don't know about you, but when I end up working a shift that is short staffed, I like knowing that I work for someone that at least tried to reach out to every single warm body that could possibly help cover the shift.

My unit has been chronically short staffed recently due to a high census and staff illnesses and injuries. Just the other day one of the long-time unit nurses was chuckling to us that she can't believe the charge nurse even asked her to pick up a shift because she hasn't picked up an extra shift in over 30 years! Moral of the story: if you can't work, just say no... they may be asking for a long time to come, lol.