I'm going to hit my fiancee's child
I am very shocked by this. Why would you hit a child?
I'll slap his face hard so I'll feel the pain he put me through.
I do not think that you are permitted to hit anyone, specially children like your fiance's child. You maybe reported to the police and go to jail. Do not strike your fiancee's child, she won't forget it and will hate you for it.
So what? I'll spend a night in jail, but it's not like I'm looking forward to being in jail anyway.
I don't think that you want to go to jail as it can cost you money for your bail or worse, you would need to go to court because of your behavior. If you have issues with your fiance's child, then talk to her but if things do not change, you still have time to call of the wedding and break up with her. Breaking up with her is hard, I know, but it is better than doing something illegal.