Should people without kids ever give parenting advice?
Pretty good chance that they were kids once, and that they've observed a lot of parenting throughout their lives. Further, being outside your current "world" they may be able to be somewhat more objective about your situation than you could be. Having two different perspectives is often better than having only one, even if the one is closer to the situation.
But this is a silly question - "ever" is an extreme that can be readily disproven with a single instance of a non-child-having person giving good advice to a bad parent.
So my sister in-law and her boyfriend think that they know a bit about raising kids even though they have none themselves just from watching other people.
"I used to have 7 opinions about raising children. Now I have seven children and no opinions." often applies. They will shut up once they have kids, and until then they'll be insufferable know-it-alls.
You have, in my experience, two options, and this goes for any parenting advice from anyone, not just people without kids:
1. Engage them. Tell them what experiences you have had that lead you to act the way you do, and the circumstances surrounding the issue so they understand that your current course of action is appropriate for the child and situation in question. Note that this will often turn into a longer discussion where they pick apart everything you say and tell you what's wrong with your perspective, understanding, or plan. It can get very distracting because they will flip from perceived problem to perceived problem without sticking to the one issue they brought up in the first place. This demonstrates that they really aren't interested in helping you, but in figuring out parenting - this is actually one mode of learning. By nit-picking what other people are doing, they are actually integrating your knowledge and experience into their memory. It's not a great way of learning since it's so frustrating for the "teacher" but you tend to learn a lot because you force someone to defend themselves, and you quickly get to the core reasons behind their actions.
2. Be polite, listen, and don't respond. "Why are you letting him eat paste? I've read that in some cases that indicates mental instability. Have you taken him to a psychologist?" Yeah, it's annoying to have someone question your parenting skills, but it's relatively easy to defuse them. "Really? Huh. I guess I'll have to look into it. The paste is non toxic." Then go back to what you were doing or talking about before.
Note that once you had kids you probably talked about them non-stop to the kidless couples, so it's only fair that they do so as well. In fact, during your initial parenting experience they probably saw that the only way they could engage your attention for any length of time is by discussing your kids. They are now trained to discuss your kids, but there's not much to talk about except how your raising them. Just changing the subject will often be enough, because they'd probably rather talk about something other than your kids, if you gave them a topic.
I'll usually do a little bit of one, then once I've given a basic explanation, move on to two to get them to stop harping on it. This works even for parents who second-guess your efforts. We have a close friend who had a girl as her first child, and by then we had two boys. Her girl was sweet, quiet, and easy to engage in reading, coloring, and other quiet activities. Our boys were rough and tumble types tearing around the house at (toddler) breakneck speed. She only had them over once, and then wouldn't come out and say it, but pretty much implied that we must be doing something wrong since her child was obviously more well behaved than our children. We pretty much ignored it, and she eventually had a boy. She later told us she had no idea that some kids just come that way. She later had a girl who was also pretty active, and she's much, much more laid back about letting the kids be kids, and will more often say, "Wow, that seems like a tough situation" than "Have you tried X, Y, or Z?"
You simply have to ignore some comments and responses.
Opinions please. Is there any way to make them understand that they don't even know what they don't know.
Every time they give child raising advice, change the subject to when they are getting married and/or having kids of their own.
Every. Single. Time.
They'll stop eventually.