Symbolism, Imagery, Allegory
Like countless other kids growing up and wanting their independence, Peter decides at a certain point that he needs a lock on his bedroom door. Initially, his mother vetoes the idea:
"That's why I need a lock on my door," I said.
"I don't like locks on doors. We're a family. We don't have to lock each other out." (7.76-77)
She thinks that they're all one big happy family, and that Peter shouldn't want to get away from his little brother. And besides, Fudge will respect his property, right?
Wrong.
Fudge gets into Peter's stuff and ends up destroying the school project that had taken him and his group weeks of work. He goes into Peter's room and cuts his hair into Dribble's bowl. When his parents see this, they realize that Peter was right all along:
[...] my father came home with a chain latch for my bedroom door. I could reach it when I stood on tip-toe, but that brother of mine couldn't reach it at all—no matter what. (7.119)
The new chain lock on Peter's bedroom door is an unspoken apology from his parents for not taking his concerns seriously before. It's also a symbol of his growing up (Fudge isn't tall enough to reach it) and his right to have his own space now.