I will absolutely give them the benefit of the doubt... I just know that I have such a people pleasing nature that if they phone tomorrow and say they have simply forgotten to give her the milk (twice) that I don't know the correct way to handle it.
If this happens, you need to ask what they intend to do to make sure it doesn’t happen again. Hopefully they will already have thought about it, but if not ask them to think about it and set a date for a meeting in a day or two to talk about it.
The thing is, if you are going to follow this route, you need to be prepared to withdraw your DC and look elsewhere. That is the only real power you have, so if you think this is an indication of a lack of care and attention that could potentially harm your DC and certainly isn’t what you want for them on a regular basis, you need to be prepared to withdraw them if the nursery isn’t up to demonstrating that they don’t find it acceptable either.
An alternative is to simply say “OKay” as your people pleasing nature wants to and only push things if something else happens too.
It’s difficult because she hasn’t been there long so it’s a bit alarming that she’s missed her milk twice in one day already. If she’d been there a few years and something went wrong on one day out of those years you wouldn’t be as alarmed. But this is early on, it might just be unfortunate and she’ll have years more of great care, or it might be down to fundamental issues at the nursery, and you just can’t tell.