is there ever a graceful way to actually inform them concerning your opinion?
I have always had a strict “no interference” policy (on account of the fact that it’s none of my damn business) but I’ve just broken this rule concerning a particular situation and I’m afraid it’s going to turn into relationship armegeddon.
Nope. Never, ever.
The only time I would ever get involved is if I thought the child was at risk (abuse or neglect). In which case, I would be very unconcerned about the friendship involved.
Even when friends ask for advice, I dodge.
Unless there’s abuse going on (in which case, inform the cops) - stay out of it.
If they’re really a close friend or relative, you shouldn’t be afraid of telling them what you think. If you aren’t comfortable calling them out on their bullshit, then you aren’t really that close.
So I did it via electronic communication and used all tools therein at my disposal to emphasize my own awkwardness and shame at even broaching the subject. They took it in stride and good humor, and asked to see the research. All was well.
So good.
Prolly never going to do it again. I was very scared.
(It happens the person who I was talking to is an occasional participant at SDMB so if you happen upon this thread: Hey there.)
My mom watches my niece 3 days a week at my brother’s house, while my brother is there working from home. My brother and SIL won’t let the baby “cry it out” and thus the baby has a hard time getting - and staying - asleep.
But, mom won’t tell them to let her cry (or if she does, they don’t care) and mom can’t let the baby cry while she’s there at the same time as my brother. So the kid will never just “cry it out” despite mom’s opinion.
They’re all pretty close, but you can’t make people do stuff with their kids that they don’t want to do. And if you push too hard it does cause bad feelings.
What you want to avoid at all cost is making it look like you’re criticizingly them to show the world what an awesome person you are via your superior parenting.
Approach it as if you’re asking them for parenting techniques. “You’re letting your kids watch Saw 3D? Are you worried that the film will give them nightmares?”
And that’s about all you can do. Bring up the topic and then give them information on why what they’re doing might be wrong.
However, if you’re not a parent, I wouldn’t even bother.
What was the issue, incidentally?
I’ve mostly found that waiting for them to ask is the best way to handle it. Luckily, since I’ve got two kids who are each slightly older than my friends’ kids (that is, one is 18, and one is 5, and most of my friends’ kids are 13-17 and 2-4), I get asked for parenting advice all the time.
Failing that, if they’re looking frustrated, I may gently say something like, “Man, that looks rough. I remember when WhyKid was in that phase…” and that’s usually invitation enough for them to ask me how I handled it. And, I will admit, I’ve sometimes said that when WhyKid never actually was in “that phase”! He forgives me for giving him a bum rap - in exchange, he’s long since been invited to tell all his friends what a crazy messed up psycho bitch I am if it gives him a graceful way to turn down drugs or a ride with a drunk driver or staying out later than he thinks is good for him. 
Without invitation, I tend to stay out of it, again, unless we’re talking abuse or neglect, in which case, fuck the friendship. ETA: And that’s not hypothetical. I did have one neglectful friend, and I told her off big time, without worrying about her liking me.
I keep my opinions to myself unless I’m actually concerned about the child’s well being.
My nephew is a bit of a handful <snort> my Bro and SIL handle him differently than I would; however, he’s not my kid so I keep my mouth shut.
If I thought he had a health condition or was behaving in a way that would hurt him I would say something and I certainly intervene when he’s acting like a bone-head around my baby. To be fair, he’s a 7 year old boy - I’ve been led to believe that most 7 year old boys are a bit of a handful and fully expect Junior to put me through my paces when we get there.
To me, the most awkward thing is when someone is talking about their parenting choices, but not really asking for advice. I had one friend who was, I thought, insanely restrictive and harsh towards her step daughter and insanely indulgent and blind toward her son. Listening her talk about her trials and tribulations drove me crazy. She was not asking for advice: every story was about how hard her kids made her life, and she wanted nothing but sympathy. After a while, though, listening silently felt like condoning and enabling behavior I found appalling, though not really abusive. I basically backed off the friendship as a result, and I started to give some advice–but by then I didn’t care if it meant she cut me off. Hell, I was eager for it.
I didn’t say anything to my sister and brother-in-law about the psychobitch* they’ve raised (with a very sweet younger sister watching and taking notes all the way - well on her way to Psychobitch II now); they broke off relations with us for a different stupid reason. 
*She has had terrible temper tantrums all her life - she’s still having them at almost 16. She never actually stopped having them. She just has more words to include in them now, like calling her mom a fucking bitch.
We (parents) do this all the time by telling other parents how we do things at our house. Don’t we? When my busybody friends tell me they they NEVER allow their children to watch TV, I understand that they are judging me because I park my kids in front of the TV for hours each day (but only so they won’t bother me when I’m drunk.) It’s cool. They are just telling me a story about how they do things and, when you have children, that is pretty much all you talk about. All the parents I know constantly worry about whether they are doing their jobs well and these types of conversations, when done gently, can be reassuring and/or informative.
Hell no.