What is the first trick Matilda plays on her parents?
The first trick that Matilda plays on her parents is putting Superglue inside her father's hat so that it sticks to his head and makes him look ridiculous when the hat finally has to be cut off.
We know that she's going to plan something as soon as the narrator tells us this:
She decided that every time her father or her mother was beastly to her, she would get her own back in some way or another. A small victory or two would help her to tolerate their idiocies and would stop her from going crazy.
Matilda is only five years old, but she's brilliant, and her parents mistreat her horribly. She decides to take revenge.
More specifically, her father has just called her an "ignorant little squirt," and her mother has told Matilda to keep her "nasty mouth" shut. These are terrible words to say to anyone, let alone a child. So you can understand why Matilda tries to fight back.
If you go to the chapter titled "The Hat and the Superglue," you can read about how Matilda reaches her father's hat, squeezes a neat line of Superglue all along the inner rim, then places it back on the peg it hangs on. Mr. Wormwood dons the hat without suspecting that anything is wrong, but then he suffers endless embarrassment at work that day when he can't take off his hat. The following morning, Mrs. Wormwood has to cut the hat off her husband's head, with the ridiculous result being that "he finished up with a bald white ring round his head, like some sort of a monk. And in the front, where the band had stuck directly to the bare skin, there remained a whole lot of small patches of brown leathery stuff that no amount of washing would get off."
Matilda's revenge is complete. She's made her father suffer, and her mother, too, has been inconvenienced and annoyed by the incident.