if you look up the original hebrew, it is:
Qalal – be of little account, to be insignificant, to be lightly esteemed, to make despicable, to treat with contempt, bring contempt or dishonor. So its not just a child who in a fit of anger curses at their parents and then gets put to death.
In fact there is no mention of it being a kid/child as the modern word would imply, it is simply anyone, or in hebrew 'iysh. Which in many other uses in the bible is used to refer to any human.
Some more context can be given with the passage in Deuteronomy 21:18-21
Deu 21:18 If a man has a stubborn and rebellious son who does not obey his father and mother and will not listen to them when they discipline him,
Deu 21:19 his father and mother shall take hold of him and bring him to the elders at the gate of his town.
Deu 21:20 They shall say to the elders, "This son of ours is stubborn and rebellious. He will not obey us. He is a profligate and a drunkard."
Deu 21:21 Then all the men of his town shall stone him to death. You must purge the evil from among you. All Israel will hear of it and be afraid.
So as you can see this isnt likely a kid, unless you were a vile, immoral (see
profligate) drunkard as a child.
Also as you can see this is the last resort for parents in situation like this, this isnt done on a whim this is done after all other attempts to correct the situation has failed (see verse 18) and with the consent of the elders and all the men of his town.
This isnt just "oh my kids a brat, guess ill kill him" it was a way for the nation of Israel to protect the fabric the their society, and was not done lightly.
You can't just take one verse out of the bible and say thats what the bible tells you to do. You have to take it in context with the whole thing, along with the cultural aspects of that time and language.