So far I have never smacked her, however a couple of days ago she bit my 8 month old son so hard through his trousers she broke the skin and caused a nasty bruise. I left the room with him (leaving her on her own which she hated). I was angry with her for about half an hour. Showed her what she did. Told her to say sorry. Her nursery teacher has had a quiet word with her as well.
The next day she pulled his hair very very hard so he really cried. Again I was angry with her. I have said if she ever really hurts DS again she will get a hard smack on the back of the hand.
She hasn't hurt him badly again (just stuff like sitting on him).
If you think my idea is unreasonable please give me an alternative strategy. I'm not sure what else to do. She gets loads of love and attention. Thanks.