I don’t advocate violence against clowns or anyone else. But if, hypothetically, one had to defend oneself against malevolent clowns, I’d think that a knife would be a poor choice of weapon. Clowns typically wear multiple layers of loose-fitting clothing, often with padding underneath, obstructing true thrust. Many clowns are accomplished magicians, far more likely than the average person to disarm an opponent in a knife fight. And many clowns are trained dancers, skilled at avoiding thrust. Since most knife fights require several successful strikes of the blade to terminate the threat from one’s opponent, those advantages make it quite unlikely that an untrained combatant would win a clear victory in a knife fight with a clown.
If this girl is correct in regarding clowns as so grave a threat to her well-being that she needs to carry a weapon, an assessment which I by no means endorse, I would advise her to carry something else, perhaps a heavy club or a spray bottle or something else where a single stroke is likely to disable an opponent. Her school probably has policies against 11 year olds carrying guns (though it is Georgia so maybe not,) so firearms really aren’t worth considering even if her threat-assessment is correct.