Cathy, when she learned the master had lost her whip in attending on the stranger, showed her humour by grinning and spitting at the stupid little thing; earning for her pains a sound blow from her father, to teach her cleaner manners
In this passage of the novel it is easy to tell that the children, as well as Miss Cathy and the housekeeper, see the child as an outsider, and as a dirty animal. They speak of how the child is old enough "to walk and talk" but that he only repeats "gibberish that nobody could understand". The child isn't doing anything exceedingly odd other than existing in a place where those around don't think he belongs. It's interesting that later in the passage, it is stated that the daughter Cathy grins and spits on the "stupid little thing", acting the most animal-like of anyone there. The father then strikes her, again showing the more wild and animalistic side of Wuthering Heights. The housekeeper says that Mr. Earnshaw strikes Cathy as a way to "teach her cleaner manners", but clearly if she spits on a toddler her manners won't be helped by being hit. The fact that the family is treating the child like a rodent that someone weaseled it's way into their home is interesting because they are the ones acting like animals.