Why does Rachel disapprove of this plan?

Rachel disapproves of their decision because the Matthew and Marilla know nothing about the child's history.... no background information. There's no telling what the boys will be like.... she warns Marilla that the child might even be dangerous.

‘Well, Marilla, I’ll just tell you plain that I think you’re doing a mighty foolish thing—a risky thing, that’s what. You don’t know what you’re getting. You’re bringing a strange child into your house and home and you don’t know a single thing about him nor what his disposition is like nor what sort of parents he had nor how he’s likely to turn out. Why, it was only last week I read in the paper how a man and his wife up west of the Island took a boy out of an orphan asylum and he set fire to the house at night—set it ON PURPOSE, Marilla—and nearly burnt them to a crisp in their beds.