The right thing is this situation was to ensure the baby's safety, and you did that. Why was it important to say she was hispanic...? because if the older little girl could have said, "yes that is my baby sister, my mommy is in the potty right now," you could have at least sighed in relief that the infants parent was in there.
I am totally not sensitive about this stuff, I live in California, where tons of people don't speak english. It's scary in moments like these when you simply can't communicate. I once had a women come up to me and several other people panicked and crying and I couldn't understood a word she was saying. She finially grabbed someones hand and dragged them to her car....come to find out her little boy got out of his car seat and the car while she was pumping gas, turns out he was ok, just in the store looking at candy.
Here's something similair that happened to me. I was walking in a grocery store and the car parked next to me had three kids in it alone. The oldest was probably 6 and the youngest was probably 18mo. I walked in, got what I needed and came out, still the children were alone. I did what you did, looked around, waited. Finially when I saw the little boy trying to turn the keys in the ignition, I called the cops. They responded right away and ticketed the father, who by the way was cursing me out the whole time, and told me that its the law as a citizen to report children who are unattended.
God forbid if that little boy got that car started and out of gear, they could have all been seriously hurt.
Now, of course I wouldn't call the cops on every scenerio that involves a child, but in that one I don't regret it. In the situation you gave I would probably stand around, wait, call the manager. You did good! =)