You know where that phrase came from? It was coined in Schenck v. United States[1], where Justice Oliver Wendell Holmes Jr convicted the defendant for publishing pamphlets opposing the draft in the first world war.

1. https://en.wikipedia.org/wiki/Schenck_v._United_States

I would like to point out (not necessarily to parent) that this was an example of speech that was not protected by 1A in a case where a man was prosecuted by the government for trying to tell young Americans that they don't need to join the draft for WW1.

Also important: yelling fire in a crowded theater is entirely protected speech if you don't believe it to be false.