There are two letters, and it is only because Vernon had the sense to snatch both of them away that Dudley didn’t have the opportunity to start reading his.
Petunia shakily seats herself, taking the letter for Dudley and opening its envelope.
Except for a line about a school representative visiting, it is exactly like the other.
Vernon is stiff with indignation that she is upset- not at her, but at the letter and its sender- and he takes hold of her hand. “We’ll tell them no, Pet.”
“But-”
“You said it’s dangerous, didn’t you? We tried our best to stamp it out of him, but it’s gone past that now.”
“They won’t take no for an answer,” she says, voice quavering.
“Well, they’ll have to these days. It’s different. We’ll refuse when they show up, and if it comes down to it, I’ll have them trespassed.” Vernon sounds so sure of himself.
In the end, they are not able to have the deputy headmistress trespassed, and as much as she hates it and Vernon dislikes it, neither of them can say no to Dudley’s rapturous desire to go when he finds out, not when he declares he wants to learn magic, even if it does mean having to go to school with Harry.
Petunia remembers how she felt and has never wanted that for Dudley, and while Vernon’s never felt like that as far as he is concerned Dursleys make decisions for themselves.
So, both boys leave for Hogwarts.