Underage drinking is something I feel I have way too much experience with. I can't speak for other's here but rather only share my own personal experiences with it.
First off, technically every single kid has had alcohol in their body. Whether is be from the vanilla extract from raw cookie dough or an overly ripe fruit that's starting to ferment. Yeah, I know that's not nearly enough to cause any form of intoxication, but you get the point I'm trying to make here. Now onto my experience.
I am almost 24 and have an almost 3 year old and 7 year old sons and I'm legal guardian of my 15 year old brother. Technically, I been drinking alcohol my entire life. My parent's remedy for anytime I was sick or wouldn't sleep was to give me hot brandy and honey. When I was 9 years old and living in a group home, myself and some of the other kids snuck out during the night with 2 of those large fat bottles of wine with the little handles on the neck. I don't remember how much of the first one I drank, but the second one I drank all myself. Needless to say I had to be carried home. So that was my very first experience with the major effects of alcohol. After my time in the group home was done and after some legal troubles I got into at age 11 and 12, I was back living with my parents again. They were pretty Conservative about certain things like enforcing corporal punishment and such, but very Liberal about letting me stay out as long as I want and allowing me to drink at home. But I was never a binge drinker during my teenage years, but rather just a simple casual drinker. Whether that's considered good or not, it's still a no-no for a teen to drink. Now the real kicker is whenever we were at family gatherings. Nobody said a word if I went over to the cooler and took a beer or made myself a margarita. One day I actually asked why nobody minded and the reply was that I drank like a responsible adult and not an idiot. I guess having that type of freedom made it feel like drinking alcohol at my age wasn't that big of a deal whereas my friends who didn't have that freedom wished they did and the rare times they got ahold of alcohol, they didn't know how to handle it when the effects kicked in.
Anyways, fast forward to the present time. I rarely drink anything around my kids and that also includes my brother. I also DO NOT give either of them whole drinks or even sips. My brother had a few alcohol related incidents with his friends, but nothing major. Just slept it off. It's easy to explain my rules about alcohol to my brother, but it took a bit longer explaining to my oldest son. See, during a dinner where I did in fact drink a Smirnoff Ice with our dinner, my oldest kept reaching for it and thought it was funny. I didn't find it funny and it almost earned him a spanking because defiance towards adults is a cardinal rule in our household that earns an auto-spanking no questions asked. So after dinner and while he was getting ready for bed, I sat down with him and told him the entire story about me drinking as a kid and explained to him that in many ways, it made me grow up too fast and miss out on my childhood. While all the other pre-teens were doing pre-teen stuff amongst eachother, I was sitting with the adults drinking beer and talking about boring adult stuff when I should have been playing with others my age. My son understood clearly and hasn't said a word about it since. I'm sure the same conversation will need to be had with my youngest in a few more years.
So yeah, I may seem like a hypocrite, but I use my own experience as a teaching tool to them. I hope this post doesn't seem like I'm supporting kids drinking based on my story because that's not the case.
Now as for my feelings nowadays about the OP of this thread. All parents are very sensitive and defensive when other parents try and tell them how to raise their kids. If I am at a family gathering or party in general and I see something harmless like a dad giving his son a sip of beer, then I'm not gonna say anything, but if I see a kid doing the things I did as a kid, I'm definitely gonna say something to the parents. Maybe even tell them my story if that helps.