At the end of Home Alone, when Kevin's family asks him what he did while they were away, he says that he went shopping and just hung around but he doesn't mention defending his house from the two burglars Harry and Marv.
Why did he keep silent about them? What would have happened if he told them?
It has been shown in all movies of this franchise that no one believes what a kid says or can do. Like the cashier at the store where Kevin purchases stuff for booby trap, or the receptionist at the hotel in Home Alone 2, or the police chief in Home Alone 3.
Kevin is a kid who constantly gets into trouble and gets punished. If he tells anybody about that, they will think he is just joking or up to some mischief.
In the beginning of the movie, when a kid eats his pizza and he spills Pepsi on the table, his excuses are not heard and he gets punished.
Kevin: He started it! He ate my pizza on purpose. He knows I hate sausage and olives...
Uncle: Look what you did, you little jerk!
Mr. McCallister: Get upstairs now.
I disagree with the other answer that it's because they "wouldn't believe him". Their entire house was destroyed. All Kevin would have to do to be believed is simply not clean up, and then involve the police, who already know of the "Wet Bandits".
One of the themes of the movie is that Kevin is more mature and capable than anyone in his family realizes. So that line was an inside joke between Kevin and us, the audience. "What happened?" "Oh just hung around (wink wink)". The intent is to keep us on the inside with Kevin. We all know he's much more capable than his family believes he is.
