Take this typical bad parenting scenario...
A child with anger issues who has a hard time controlling their temper. He has frequent tantrums, almost daily, they last for hours. Parents send the child to his room where he gets worse and worse. The parents think they are leaving him to calm down but they are just isolating him and making him feel unloved. The child loses control of himself, and the parents have no control over the child. They try to punish him by smacking, usually leaving a mark. Sometimes the smack becomes a slap round the face. This angers the child more. The child ends up feeling very unloved and unwanted, even though the parents frequently tell him they love him, their actions show him otherwise. The parents openly refer to the tantrums as 'toddler tantrums' to other family members, making the child feel ashamed.
Clearly the parents are desperate and ill-equipped to deal with the meltdowns. They clearly love the child, but their actions make him feel ashamed and unloved.
My question is, where does the line get crossed from bad parenting to child abuse?