Haroldthecat - also here to try to offer support to you. I think we should all stop replyto Mrmanager (hope he is a man and not a woman. I was a social worker in childrens services for almost 30 years (recently retired) and I can assure you that SSD will not "take your child away" because you are shouting at her and smacking her (eventhough it is against the law and legally anyone who assaults a child (even by smacking) could be charged with a S47 assault in the same way as if you went up to an adult in a supermarket and "smacked" them.
I worked with foster carers who cared for very very troubled children with all kinds of behaviour problems and they were all told that smacking was not allowed. However we also said that if they did do it in temper, would they please tell us, so that we could re-assess the placement and help the carer to prevent this kind of thing happening. We did not hold with carers who said they smacked in "cold blood" as it were, as this seemed to us to be far worse, that losing your rage and slapping (it's called being human!)
As most people have said you wouldn't be posting on here if you thought what you were doing was ok. Can I ask has your little girl only become "naughty" since the new baby arrived. After all she has presumably had your undivided attention for 5 years. I agree absolutely with Chipping IN. Your daughter is giving you a clear message with her naughtiness - it means "I'm jealous, I'm frightened you don't love me any more, I want my mommy back and that baby to go away"
Negative attention (for a child) as often seen as better than no attention. Seems you are in a vicous circle, DD thinks you don't love her anymore, only the new baby, so she gets more and more wilful etc and then you shout and smack and then she is sure you don't love her and becomes worse and round and round you go in this viscous circle.
Agree with everyone who says you could have PND or just be exhausted with the demands of the new baby and your DD's reaction. Be brave and talk to the HV (be honest) and ask for help as others have suggested. Your DD needs positive parenting, ignoring as much as possible the unacceptable behaviour and praising (in a fairly dramtic way) any good behaviour "WOW HOW good are you my best girl"
etc etc.
Things will start to level out, your DD will get used to the new baby and getting less attention, the new baby will be less demanding and you will be less tired and possibly depressed. WELL DONE for coming on here and admitting something you know is not on - and it isn't on - It HAS TO STOP, but you need support. Please don't (as someone suggested put your DD in after school club) she needs more time with you not less.
Let us know how you get on......and sending you good wishes.