The comment about alcohol is kind of what I was getting at with my long essay about stuff EVERYONE agrees is bad or inappropriate for children (cigarette smoking, X-rated movies, bullfighting, sex), stuff that everyone understands is controversial -- you might give it to your OWN kids or let your OWN children participate in that activity, but not someone else's without talking to their parents (R-rated movies, sips of beer, violent video games), and stuff that is widely perceived as good harmless fun (Disney movies, soda pop).
When you ban something that the rest of the world considers good clean fun, you're swimming upstream. It gets a lot harder if you try to enforce that ban when your kid is in someone else's house. And it also gets a lot harder when YOU eat/drink/do whatever it is you don't want your kids eating/drinking/doing. It just does. Of course, it's your choice and as a parent, you have every right to drink a Coke every day while not ever letting your kid have it. But since most other parents permit pop as an occasional treat, your child is likely to come to resent it and see you as a hypocrite (maybe not at the age of six, but later on) in a way that he is unlikely to resent it if you drink a glass of wine and don't let him have any. Fair or not, it's just the way things are.
I told my dh about this conversation and he pointed out something that I think is worth noting: when we were kids (the 70s and 80s), pop was much less of a kid's drink. Our parents drank pop but we didn't get it routinely and neither did our friends. No child we knew got to drink pop with dinner, though plenty of parents did. When we got pop, it was a special treat and usually a very small quantity. (I remember two things in particular: when my family would stop at McD's for lunch on road trips, they would buy one large orange soda and divide it between me, my sister, and my brother. And when we had to take some sort of foul-tasting medicine, we were allowed to wash it down with a shot of R.C. cola, my mother's prefered brand.)
But whenever I think about this thread, I find myself putting myself in the OTHER mother's shoes. My daughter Molly is only 20 months old, but let me pretend that she's six and I've taken Molly and her best friend Maddy to the zoo. Now, although dh and I don't stock ice cream on a regular basis (because dh binges on it given the opportunity), we regularly buy Molly ice cream when we're out somewhere like the zoo, as a treat. Of course, since Maddy's along, we buy her an ice cream too, because you just don't give your own child a treat without getting something equivalent for their friend who is also along.
The next day I get a call from Maddy's mom. "Please don't buy Maddy ice cream," she says. "We've been telling her that it's something just for adults, and we really don't want her to ever have it."
"Oh gosh, is she allergic to dairy?" I say, imagining hives or worse.
"Oh no, nothing like that. We just don't want her to eat anything with refined sugar in it. No ice cream, popsicles, soda, cotton candy -- no junk food."
"So when I take her to the park and buy a treat for Molly, what can I buy for Maddy?"
So the next time we go to the zoo, do we deprive Molly of whatever treat she expects? Do we buy her a treat and eat it in front of poor, deprived little Maddy? As a hostess, this is a real dilemma. I think in this situation I would talk to Molly and ask her if she had any ideas for solving the problem, in the hopes that she would say, "Well, I really like Maddy a lot, and I don't want her to feel left out. So I guess we can just drink water when she's along." Another solution would be if Maddy's mother suggested an alternative treat -- ice cream was not okay, but popcorn was, or a snow cone, or whatever.
Children have a pretty good instinct to be hospitable, and they really like to share treats with their friends. When I was in grade school, there were Jehovah's Witnesses in my class who weren't allowed to celebrate birthdays, Valentine's Day, etc. So when I would bring in gingerbread cookies cut out in the number 9, since I was turning 9, I would also bring a gingerbread cookie in the shape of a duck, for the JW kid (and apparently that was enough of a dodge for their parents to accept, because I remember the ducks being eaten). On V-day, I might bring in 25 heart-shaped cookies, and one duck-shaped cookie. At Hanukkah, 25 dreidl-shaped cookies, and one duck. (What can I say? I liked the duck cookie cutter and I liked baking gingerbread.) There was also a kid in the class who was lethally allergic to peanuts, and everyone was always very careful to bring in an Adam-safe treat as necessary. Another year there was a gluten-intolerant kid -- again, we always tried to bring in something for him, too.
The key thing, though, is that we really wanted to include everyone. If there's something your child isn't allowed to have, for ANY reason, it's a good idea to suggest alternatives. "I'm sorry, we don't like her to drink pop. But she really likes juice, and juice is a great treat for her."