Look at these two "moms" who are actually models that probably don't have kids. Look how happy they are, sipping wine and talking about the joys and perils of parenthood. But it's not always funny when your friend, sister, brother or even your own child, is doing a not-so-good job of raising their child.
Okay before we even through out the term "parent-judging", remember we all judge and many times we should judge. It's essential to our well-being as a society, and it can be positive as well as negative. Example: When Steve from my show has his daughter call me on the phone herself to sell me Girl Scout cookies, rather than bring in the sheet and sell them for her, I admire that and I tell him.
If you were at the mall and saw someone dragging their screaming 4 year old around by the hair, while stopping occasionally to slap her, you'd judge. You should judge. You'd be doing the right thing to call 911. (BTW, I wouldn't call mall security because I have no idea whether the security guard to responds has been trained to deal with a situation like this)
But before I get too far off track, let's get back to being annoyed at a friend's lack of parenting skills. 20 years ago, I knew an awful young couple that had a baby they didn't know how to take care of. They were out of formula once so they gave the baby Mountain Dew. You can't make this stuff up. They had the money for formula, but it meant getting in the car and driving to the store, so Mountain Dew was a better choice.
I'm going to guess most of the time, your annoying friend/parent isn't that bad. But if you do get annoyed with their parenting style, here's a great article from parents.com.