Or what? Or else? Or you're grounded? Or you're in trouble? Or you're going to time-out?
Threatening a child is ineffective parenting and generally not a good idea. It teaches your child that using brute force or superior cunning to get what they want is an acceptable way to deal with someone who isn't doing what they want them to or cooperating.
It also creates an awkward position for you, the parent, to threaten, because now you either have to follow through with your threat- which means exacting a punishment you invented in the heat of your anger, (or as I would call it, in the throws of your tantrum), or you have to back down because the threat wasn't real, which teaches your child that you don't mean what you say. On one hand it's great not to follow through with a threat, but on the other hand children learn about honesty, truthfulness and trust from us. Saying what you mean, and doing what you say are KEY to your child developing those character traits.
What can you do when the threat escapes from your lips and you immediately wish you could take the words back and start over? While you can't start over or erase the words you said, you can calm down and get to your child's level to talk about it. "I'm sorry I said that. I was angry and didn't think about my words before I said them out loud. It was wrong of me to threaten you. It is not respectful or kind to threaten people just because they don't do what we want, and I wasn't being a very good friend to you when I said that. Now that I've calmed down, here's what I really meant to say..." (read on for suggestion).
Whether you forfeit on your threat or follow through with it, you still won't get the result you want as it's likely to happen again at a later time, and worse, you are damaging your connection with your child.
I love to use the employer/employee analogy to help parents really understand how their words and actions affect their child. If you had an employer who constantly threatened to fire you, or discipline you, you would be in a constant state of stress and anxiety, you would not like your employer very much and definitely would not consider them a friend or someone you could trust. As an adult, you could quite working for that employer, but a child has NO such recourse when they are threatened, and treated unfairly and disrespectfully.
Yes, it can be difficult to resist that knee jerk reaction to threaten. Respectful and gentle parenting is about responding versus reacting. It takes practice and effort at first, but it really is worth it!
Try redirecting to something more appropriate instead. "I can see you are feeling angry and frustrated. It's not OK to hit your brother. Hitting hurts. I'm worried your brother will get hurt, or he'll retaliate and hurt you. If you'd like something to hit to help deal with your anger, you may hit a pillow, the couch or the bed."
When you offer an alternative that is safer, you are still allowing your child to express her feelings, and you're validating her emotions even as you set clear boundaries for her behavior. This ultimately leads to better self-control and emotional well being for your child, and sets them on the road to showing respect and kindness to others.