"Well, Tommy, it all started when Helga Hufflepuff decided to try to defuse an argument between Salazar Slytherin and the other founders. At the time he was insisting that Hogwarts could not handle the influx of muggleborn and muggle raised students, and that they needed to be turned away for the good of the school. So Helga just took him aside and asked him straight out, 'Salazar, seriously, what do you have against those poor children?'"
"And Salazar said something like, 'It's not that I'm not sorry for them, really. Over half of them can't read, most of them are abused by their families, some of them horribly, they're poorly-fed, sickly, sad little things. It's just that they need so much! We need something more than just Hogwarts if we're to keep taking them in. When we try to put these children in the same classes as our own, we do both groups a disservice and we place Hogwarts in danger from some of those aforementioned terrible families. Not letting them into the school is just the simplest, cheapest solution. I have others, if people are willing to put in some effort.'"
"Seriously, Salazar? Effort is what I live for. Now, lets discuss these 'other ideas' of yours..."
"And that, Tommy, is how we came to adopt you, and why you and Charlie will be going to Hogwarts together in the fall."