This is going to be less of a blog post and more of a "go read this other blog post right now because it's fantastic."
Go read this. "Dear parents, you need to control your kids. Sincerely, non-parents."
I cannot say anything better than what he said here. Sadly, I think I used to be one of those people. Who rolled her eyes and thought "Why can't that lady get her kid under control?" or "I don't see why she doesn't just step outside until her kid stops screaming." Boy have the tables turned. I've been that lady in the grocery with her kid that is just FREAKING OUT because I didn't let her get the giant box of fruit chews. And he's totally right. As idealistic as it sounds for everyone else around you, you can't leave your entire cart full of groceries that your family needs for the next two weeks just sitting in the aisle so you can get your toddler under control.
He didn't mention it in his blog, but I'll mention it here. It's not just non-parents either. Admit it. You've been in the grocery with your own child and witnessed another mom with a toddler mid-epic meltdown. You've felt sorry for her, but you've also silently judged "Thank goodness my kid doesn't act THAT bad."
Parents, non-parents, let's knock it off. The mom in the grocery or in church or in the waiting room is trying her best. She doesn't want her kid to be screaming any more than you do. Cut her some slack.