What's the funny thing in this joke?
I read a joke in Reader Digest which goes like below:

On his birthday, my husband was stuck driving our six rambunctious children around. As usual, they were yelling, punching, and annoying one another. Joel finally had had enough.

"Kids," he said over the din, "if you would behave and be kind to each other, that would be a very nice birthday present for me."

Our six-year-old shot back: "Too late, I already got you another present."

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I can't understand what the kid says and what's the funny thing about it.

Question

What's the funny thing in this joke?
I read a joke in Reader Digest which goes like below:

On his birthday, my husband was stuck driving our six rambunctious children around. As usual, they were yelling, punching, and annoying one another. Joel finally had had enough.

"Kids," he said over the din, "if you would behave and be kind to each other, that would be a very nice birthday present for me."

Our six-year-old shot back: "Too late, I already got you another present."

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I can't understand what the kid says and what's the funny thing about it.

Answer

The kid got his dad a present for his birthday, so he just said he's not going to behave and be kind to his siblings because the dad is asking that that would be his present but since the son has no need to give him that present because he had already bought him another one, he won't do as the dad asked. 
"too late, I already got you a present" = " I don't need to do what you said for it to count as your present because I already got you another present" 
I hope I've helped. :)

Answer

Little children can't understand  the concept of silence as a gift/ present to their parents.  At that age, they can only understand  actual  real objects  as  presents.  So,  "Too late, I already got you another present," means that the 6 year old misunderstood that the father said peace and quiet would be "a present/gift"  for him. Since the kid  had already bought his father a gift, he did not want to get him another one.