Dealing with broken promises

My daughter is 4 years old and her bmom lives about 7 miles away. We have had an very open relationship and we have spoken openly about parenting expectations from the beginning. Her life is changing now and I can appreciate that but as my daughter gets older it is harder to explain away times that C. says that she will be here and doesn't show up. I mentioned this to her casually after she didn't show up at her birthday party. I really don't want to make her feel guilty and I don't want to lay a burden at her feet - I just want to protect my daughter as much as possible from these let-downs. On Savannah's birthday C. had her boyfriend call and say that they wouldn't be over until after he got off work and could drive her (she just had a baby the week before) I completely understand if she didn't feel well enough to come over but she didn't even call S. and let her know she wasn't coming which left me being the one that has to explain why I said C. and sissy would be here and they didn't come. Most of the time I don't tell my daughter that we're getting together until I'm positive that it will happen because too often she flakes out on me. I always want Savannah to have a good relationship with C. and her kids but I am concerned that she will just become more and more disappointed as she gets older and has more understanding about C. not following through. I'm not expecting her to have continuous contact - I'm happy with what ever she is comfortable with - I'm just not happy with broken promises. Anybody gone through this before? Any advice?