When you become a parent, you become somewhat of an unsolicited advice magnet. It’s not that people are trying to be rude or condescending or vindictive, it’s just what people do. Sometimes non-parent friends aren’t sure what to talk about, now that you’ve got a bun in the oven or a baby in your lap, so they try and involve themselves in your parenting. That’s totally fine, to be sure, but there’s a fine line between being a thoughtful friend and being, well, not, and there are a lot of things that non-parents should not comment on. Not from a place of "expertise," at least, and not if it's challenging what one parent has decided works best for them and their children.
Technically, there are certain things that no one should be commenting on when it comes to your personal life and how you raise your kids. Of course, there's flexibility and room for comment and life isn't black and white enough for the collective "we" to say, "You can say this, but you can't say that." For example, as a new mom, you might want to (at the very least) have an open mind when your veteran mom friends make parenting suggestions or give warnings about certain things. Then there’s your own mom. You might want to ignore some of your mom’s parenting advice while totally embracing other gems of wisdom. Finally, and it's definitely worth mentioning, there are moments when you should listen to anyone's comments on your kids, especially if it has to do with their safety or their health or an issue you may be overlooking (because you're a human being and you can't possibly see everything all the time.)
However, minus a few select and unique situations, people who don't know what it's like to be a parent (no matter how often they've watched children or worked at a daycare and/or how many siblings they have) shouldn't be commenting on certain things, including the following: