RSBM
To be frank, we don't know that she never abused the other kids, just that Quinton was the one who died. There would have been a reason she didn't have custody of two of them. Whether it was that her unstable life and drug use made her unsuitable, or whether it was because she'd slapped them round before, I don't think we'll find out unless it comes up at trial, since it concerns minor children who are still, thank goodness, alive to grow up.
Even if Quinton was the sole focus of her rage and was the only one physically harmed, that environment is still abusive for the others. If you see your little brother being beaten, you're just as powerless as him to stop that. You're still emotionally damaged by it. You just don't have bruises on the outside for people to see. Kids who are 'favoured' by not being beaten, by given special treatment, toys, affection, while their sibling is physically abused, neglected and shunned often have just as many emotional and psychological problems later in life. It's partly survivour's guilt, partly a feeling that they were responsible or could have prevented it. No child is responsible for the manipulation and mindgames abusers play to divide siblings from each other.
But none of this may be applicable in Quinton's case. If it goes to trial, we may learn more, but if she pleas out, we might never know the details of the circumstances that led and contributed to his untimely death.
Very much my opinion only.