Castle Blair by Flora L. Shaw
They also have little inherent moral sense, especially the eldest boy, Murtagh, whose temper almost causes him to have the estate manager killed by one of his non-wealthy friends.
The poor children are there for the amusement of the rich children. They even tell Teresa at one point. Murtagh declares that they will "protect" her, and his sister Winnie agrees, saying: "Why, ye live on our land, don't you? So we're bound to protect you even if we didn't want to." (p. 55)
I'll be thinking about this one for awhile... there could be a paper in this, if I wanted to pursue it, following up on the book chapter about how adult approval of fiction for children correlated with the social class of the children depicted in the story.