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.]
You: "Nanny nanny boo boo, stick your head in doo doo!"

[Subject does or says something to offend you.]
You: "Up your nose with a rubber hose!"

[Subject calls you a name.]
You: "I know you are, but what am I?"

[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.]
You: "Make me!"

[After you say "Make me!" subject says the following.]
Subject: "I don't make trash, I burn it!"
You: "So pick up your ashes and leave!"

[Subject assails you with an insult.]
You: I'm rubber and you're glue. Whatever you say bounces off of me and sticks to you.

[Someone does something naughty.]
You: "Ummmmm!"*
* Note: Raise pitch slightly like an air raid siren. Sustain for about three seconds.

Subject: "Where's the beef?"
You: "Between your teeth!"

[Subject asks you any question.]
You: "Yo mamma!"

Subject: "Smart *ss!"
You: "It's better than being a dumb *ss!"

Subject: "How are you feeling?"
You: Put a puzzled look on your face and say, "With my hands, as usual."