I typically only serve milk after lunch, water the rest of the day. And if we go out, the parent pays if they're able. One of my guide lines I have to follow is that I can't tell the parents they have to pay for an outing. I can ask them to "donate" to an outing, but if they don't I'm responsible for paying for that child.
I asked all my families before our first outing, and they all told me to charge them. I think it helps that they're teachers, so they are used to parents paying for field trips.