Milk carton Laddie but I don't think the boys took him.
I think they found him.
I think Laddie was kidnapped in the horrible, more typical way, and the boys, or possibly Star, found his discarded body. But he hadn't quite died yet, and that's why they changed such a young kid.
It doesn't make sense that they would, on their own accord, decide that they just had to have a really little brother. Laddie's age stands out amongst the group of them, he hangs with them, and loves them, but body and mind he is younger, and wouldn't be able to keep up with their pace as easily. Just, logistically, logically, it's more difficult having such a younger, smaller person around, who would need things so much different than the rest of them.
But they love him. They make those allowances for him. They dont act as though having him around is burdensome, or unwanted, or just something they're doing for no reason. They look after him and take care of him and make sure he doesn't trip or fall, and that he (and Star) have nice, comfortable places to sleep. He has a stuffed animal. They include him, but they still shield him from the heavy, intense side of vampirism.
They didn't plan on turning Laddie, and they didn't hunt him out.
They rescued him.











