...there will be times when you're "that parent" with kids who are struggling (very loudly) to abide by social norms. That's okay!
They are behaving according to their development, and any adults around judging a parent who's doing their best to manage a struggling child are the ones behaving badly and deserve a time out. Unfortunately, those adults are also behaving according to their development.
...don't give in to unacceptable behavior, but don't give it opportunity to damage your child's self worth or your relationship. Harsh punishments, harsh words, or having your own adult temper tantrum, can cause problems that aren't so easily grown out of. With patience, love, work, emotional maturity, and maybe even professional help, it will get better. So stay calm, let it run its course, and remember, do not negotiate with terrorists.