1. Take a breather from the kids.
2. Consider going for behavioural classes or therapy. KKH has this at their child development unit. It may sound drastic and expensive but it isn't necessarily the case.
3. Do not be violent to them. Children emulate behaviour. You slap them out of anger, not just discipline, they'll think it's ok to do the same to you when they are angry and it spirals out of control.
4. If they misbehave in public when they are on an outing treat, where possible, stop the outing and head home immediately. That in itself is punishment to them. My parents did the same to me and my brother, we learnt eventually to behave in public.