I agree with the others. I can be flexible with other things but payment is not one of them. Just as she needs to pay her hydro bill you have bills of your own. If she doesn't understand that she is not a parent I would want in my daycare. Maybe she should tell her story to the power company and see what they say? A daycare bill is no less important than any other one and it's on her (not you!) to budget for it.
Also, I know parties can be expensive but it sounds like she has her priorities in the wrong order. A 2 year old would be perfectly fine with a party that has nothing more than a homemade baked cake, a dollar store book and lots of hugs and kisses from family. Unless you only charge $5, that excuse wouldn't sit well with me. If you have the finances by all means throw an elaborate party but don't tell me you can't pay your dc bill because your having a party.