Because you are such a clever parent!
It's not easy to discipline kids sometimes because you have to be on top of it every step of the way. Make your kids aware of your behavior expectations and try to be consistent about enforcing that. And when all else fails, be ready with these quick-fix ideas.
1. SECRET CODE
One night, when you have everyone's attention, let your kids know that you don't want to embarass them in public anymore than they want you to. So, in order to help them chill out if you don't like something they are doing, (and you want to help them stay in check) establish a SECRET CODE so if you use it, they will get the message. No child wants to be "called out" in front of peers or in front of anyone for that matter. Pick a word or a phrase and establish it as your code. Make sure your kids know the code and understand what it means if you use the code.
2. GIVE A VISUAL REMINDER ABOUT LOWERING VOICES
Sometimes kids can be so loud in public! Because you don't want to have to shout across a room, "Inside voices!" you need a tool. Let your kids know that if they get loud in public you will signal to them by lifting your hand above your head (showing your child that he is "up here"), and that you need him to bring it down "here," (lower your hand below your hip). Again, they will appreciate not being called out in front of others.
3. LOWER YOUR VOICE TO A WHISPER
Yelling gets you absolutely nowhere and only leaves you in a rotten mood, so give it up. Try whispering to your child if he's doing something you don't approve of. Just try it. You will need to get very close to your child. Whisper, "I don't want to yell at you, so instead, I want your such-and-such behavior to change." It's a brief connection that can work well as long as you don't do it too much. It may not feel as satisfying as just shouting your head off, but it absolutely will feel better to you and your child in the long run.
4. TRIED AND TRUE COUNTDOWN
Seasoned moms and dads know this one really, really well, as do school teachers (if they're not shutting off lights or clapping their hands, that is). Counting down is a direct cue to your kids that their misbehavior must stop. Kids are smart though, so keep the countdown brief. Use a 10 count. Speed it up if you have to, or slow it down if that works better and you see your child is trying.