I'm the last guy to ask about etiquette but the first to ask how to side step social expectations.
Ask for milk.
It makes the food taste really good and it's so out of left field but still reasonable that you aren't seen as a child, and you might even be seen as intelligent if the crowd you're with is ignorant enough. Even if it's not on the menu, ask and they'll humor you.
Although take this with a grain of salt. I am flat broke. Only done this like 3 times in my life, but I did learn this from someone much older than I with much more experience in with "social" stuff. Granted this advice is from from almost a decade ago, so proceed with cation, but if you were going to follow the advice of a stranger on the internet, you already would've been cautionary. Good luck to whoever needs this.