After they told me which station they wanted to get to and from, and the fact that they probably would return during the height (depths?) of rush hour, I figured out the round-trip fare for each member of the four-person family, advised them that each person would need their own farecard and showed them how to use the machine to get all four separate farecards at once. They thanked me effusively.
This kind of thing reminds me of when I used to work as a computer lab consultant in graduate school - basically I helped people with their computer problems. Not everyone seemed to understand the concept of "off-duty" or even "this is a dining hall or an office, not a computer lab" and kept coming to me for help. I had to tell most comers (when I was off-duty of course) no. Not all or even most of them took it well.
So I guess I carry that kind of aura. That said, I'm much more receptive to helping those who, like that family in the Metro station, ask nicely and without a sign that they feel "entitled" to free help.