I think with shots like this, if you capture their character then that's what really matters. Sometimes you get lucky with the composition and the more you practice, the luckier you get. But a lot of the 'classic' grab shots are all about the moment, not so much about the technical aspects of composition. You chose a day with nice even light and I think the snow and clothing puts emphasis on their faces, which I think is a good thing.
I think you have a good shutter speed and aperture. I imagine that lens is not the fastest focusing which will make things a little more difficult, but you have to deal with that - be in continuous AF and take bursts. Then get used to deleting a bunch of shots (or not if you have disk space).