You did the right thing telling your dad, and he did the right thing in turn by contacting the relevant authorities. I don't know you or that child, but thank you. You could have ignored what was happening, decied it was "none of your business" as so many people unfortunately do. But you didn't, and I thank you.
No child should be slapped, dragged across the floor, threatened.. that is abuse, and it is wrong. It is illegal to do this to an adult, it would be considered a case of violent assault and battery - with the cigarette lighter adding in a range of potential additional charges (torture being the main one I think). It is absolutely reprehensible for any adult to do this to a child of 5.
My own parents used "discipline" techniques that are rightly illegal now. My mother knows full well that "a little tap" is out of the question for her to give to her grandchild. She can't go back in time and change her choices as a mother, but she can effing well do better as a grandmother now that she knows better. And she does know better. Violence is wrong, and the only thing adults hitting children accomplish is that the child realises people they trust can do things to their body and hurt them whenever they like.. something that is hardly a life lesson most people want children to be learning..