That is the issue, not free speech. These were elementary kids.
It's not a matter of freedom of speech in any case, even if there were no kids. This is about an action she took while working in her position as an employee; therefore, the employer has the right to fire her for it.
If I worked for FedEx and delivered a package to Hillary, I'd probably take the opportunity to tell her a few things. I'd be justifiably fired for it, but it would be worth it.