There are plenty of things a kid can say to deflect an insult or just generally annoy any human being in the vicinity.
While not really "witty" in that they were passed around and reused ad nauseum, these little quips seemed to provide endless mirth to those who uttered them.
[You do something obnoxious.]
[Subject does or says something to offend you.]
[Subject calls you a name.]
[You and subject have been bantering back and forth for a while over whether something was said or done. You tag the word "infinity" to whatever you say to imply that your position is correct and unflappable.]
[Subject tells you to do something you don't feel like doing.]
[After you say "Make me!" subject says the following.]
[Subject assails you with an insult.]
[Someone does something naughty.]
Subject: "Where's the beef?"
[Subject asks you any question.]
Subject: "Smart *ss!"
Subject: "How are you feeling?"
Subject: "You got the time?"
[Subject tells you to do something.]