When I see someone (young, old, and in-between) do something like those girls did, I usually offer up - with a smile, "Excuse me, I believe you dropped something" (or "you forgot something" if they leave trash behind intentionally) and point to it. Most of the time, they'll accommodate the 'request'. If they don't, sometimes I'll say, loud enough for others to hear, "Here, let me get that for you". Maybe it a bit of me being a jerk about it, but I believe it'll at least make them think it through a bit the next time they do it. But then again, maybe not........
It also kinda makes me wonder a.) were they raised in a pigsty, b.) who picks up after them at home, and c.) just what DOES their home look like?
Bill