At the time it happens ignore them. They like any reaction they can get be it positive or negative.
But after the fact, make sure other adults are aware of it and ask them to deal with it when they can. Sometimes there's nothing to do but to ignore the idiots, but sometimes there is.
A while ago a friend's 13-year old daughter started receiving cat calls from a construction site that was near my last house. We went and spoke to the foreman and it stopped. They even apologized because they didn't know she was 13 (she looked older) and that was the end of it.
It won't always be that easy, but this is no case of "missing" their families and these men are simply being untoward and disgusting.