"Random Sample", a short story by T. P. Caravan; first published in The Magazine of Fantasy and Science Fiction, April 1953, available at the Internet Archive.
The violence and sadism of two children incite aliens to destroy the Earth.
The story is told in the words of a selfish bratty little girl
If you don't give me another piece of candy I'll cry. You'd be surprised how loud I can cry. Mother wouldn't like that.
to a psychiatrist:
My father says to get all I can out of you, because all you Viennese head-thumpers are quacks. He says you cost an awful lot of money. He says only an old fraud would have a beard like a billy goat.
She is not alone when she meets the aliens, she is playing with her brother:
My brother Johnny and I were out in the back yard, stomping ants, when the space ship came down. It's fun sometimes to watch ants, they run around so hopefully going about their business, carrying little bits of twigs and things in their mouths; and they don't even seem to know you're there until your foot just about touches them. Then they run away, waving their feelers before they squish. But the big red ants are the really good ones. You can jump right spang on them and they don't even seem to notice it. I guess they sink into the ground a little ways, because if you pound one between two rocks he squishes without any trouble. They taste funny. Once Johnny saw a red one fighting a black one and they kept on fighting until he burned them both up with his magnifying glass.
The girl is 9, her brother Johnny is 10:
Will you buy me a magnifying glass if I tell you about it? I'd just love to have a magnifying glass. I bet the ant thinks the sun is spread out over the whole sky. I bet he thinks the whole world is burning up. I bet it hurts, I bet I could burn up more ants than Johnny can, even though I'm a whole year younger. He's ten.
The kids make a poor impression on the aliens:
They got Johnny off in a corner, and before you could say boo! he was telling them about all the people he'd killed in the war. He wasn't really in the war, of course, but he likes to pretend he was. He likes television best when they kill lots of people. I don't think they really knew what he was talking about, but they looked as if they did. He's a very good actor.
I suppose they thought we were grownups; they were pretty much the same size we are. Anyway, they paid a lot of attention to him, so I went over and punched him a couple of times. I'm afraid we broke up the insides of their space ship a little.
They looked pretty mad. I guess they were disgusted with Johnny; a lot of people are. I always try to make a good impression on strangers, even when they don't give me any candy, so I took some of them outside and showed them how to stomp ants. It was very funny. One of them got sick. Johnny and I were still jumping up and down, stomping ants, when they took off. I hated them. They were nasty; they didn't bow down to me.
Father says not to take up too much of your expensive old time. He says no honest man could afford a penthouse for his office. You have a very nice view, don't you? You can see all over the city from here.
My, isn't it hot? I wish I had a refrigerator to keep my candy in.
Look there. Look at the fires springing up across the river: Aren't they pretty? Look. Look. And some on this side.
Take me away from here. It's too hot.
Look at the sun. Look at it. It's spreading out over the whole sky. It's burning up the city. Billy goat, help me! Save me. I'm sorry I was bad.