I think it's both a cultural difference between the UK & the US as well as a bit of the WDW magic rubbing off. I'm sorry that it sometimes make you feel awkward, though! That is one thing that worries me a bit about approaching strangers--even to be nice--because you don't know if they're introverts, or in the middle of something in life, or if they'll take it much differently than I mean it.
However, on the whole, I do feel that we, as a society in general, tend to always take the time to express when we're annoyed with someone or angry with someone, even if we don't know them, yet very unlikely to express positive thoughts to a stranger, so I have been trying to make a conscious choice to make more positive comments when I can. Although I do try to make it a very quick comment...you know walking by someone and telling them "Oh my gosh, I love your dress!!", or "Your kids are so sweet to one another! My kids act like they want to kill one another most days, so I love seeing kids being kind to one another.", and give a wry smile and then walk on.
And I have received several compliments over the years about my kids in restaurants and stores, too. I am not part of a same-gender couple, nor are my kids' racial appearances any different than mine. So I don't think that compliments are being made to other people's kids because of either of those reasons. I think you all just have cute, well-behaved kids!
I like to try to say nice things to everyone...whether they or their family look like me/mine or not, so I sincerely hope than any time I compliment someone (or their kids), that they understand that it is truly just because they/their kids are the ones I happen to see being cool/kind/cute/awesome humans that day, with no further meaning at all.
