To me heroism means doing something that requires bravery, for the benefit of someone else. It's an especially apt term for describing acts which jeopardize personal safety for the safety of others.

Would the outcome have been less tragic if they'd been more strategic, probably yes. Don't insult them by saying that they probably didn't know that is dangerous to walk on treacherous ice. They could see the kids floating away and bravely put themselves in danger to try to rescue them.