If any of them was out with his family, I definitely wouldn't bother him. If I got some sort of signal that it was okay to approach I'd go over, but otherwise I'd just smile and respect their privacy.
I have a tendency to say really stupid obvious things when I meet people I admire. For instance, I got to meet Salman Rushdie a few years ago after a lecture, and I managed to come out with the brilliant statement: "I really liked your lecture. It was really good."