Wow J., this is tough for you. First, you are beyond sweet and kind for first and foremost caring for this girl.
I think you already know that something must be done. If it was me, I wouldn't confront her or the father directly, they are neighbors so this would be awkward forever, and honestly probably wouldn't make much difference.
Documenting might be fine, but I'm guessing you are just doing the daycare thing as a favor and for some extra money, so you aren't a licensed daycare provider. Which means you don't want social services looking into what you do (not that you are doing anything wrong, just that you don't need an extra headache).
I would absolutely make an anonymous call to CPS. Let them know that they should visit the home after 5 when mom is likely to have been drinking, or when they leave your house.
I hate making people's lives hard, and I always feel bad. But I think of it this way. If you learned that mom, dad, and/or baby were killed in an accident because mom was driving drunk, you would never forgive yourself. It's as simple as that.
Please make the call, and please let us know what happened. Good luck.