Yes, well, the trouble is, "parents" tend to be super-touchy, so they're not likely to listen to anyone who uses the word "crotchfruit," even inside a book. (I say this as a devotee of the Bratfree website: I would never assume that any individual from there uses foul language OUTSIDE of the site unless he/she claims to do just that.)

It's usually best, especially in face-to-face dealings, to express yourself in terms of concern for the OTHER person, if you really want to get results that satisfy YOU. Anyone in sales understands this.

I don't use it to the actual "parents" -- mainly because I am not a total idiot.

In fact, I didn't say anything at all the other day, because I could see that it would likely not be useful.

Do adults in the US speak quietly in restaurants? Not that I'very experienced. I can barely stand the adults let alone the children.

N. Depends on the restaurant. I've heard that some places provide acoustics that make the place LOUD so that everybody thinks everybody's having a great time.
In addition, if the place is really quiet, you have to worry about the next table hearing your conversation. Depending on content, of course.

"I've heard that some places provide acoustics that make the place LOUD so that everybody thinks everybody's having a great time."

That may be one motivation for all of the hard surfaces in restaurants these days, but another one is that it's easier to keep clean. Carpet and upholstery, which used to make restaurants more pleasant places, have just about disappeared, because owners are no longer willing to pay the cost of cleaning. So now you have to shout in order to be heard across the table.

I will add that, once not so long ago, restaurants and other places where adults gathered had nurseries, where young children could be left for an hour or two while the adults entertained themselves. (I remember a particular one at a bowling alley that my parents frequented.) Thus it was not necessary to take the children into the adult space where they could be a disruption. All of this disappeared decades ago, thanks to "stranger danger" and liability.

I don't use it to the actual "parents"
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I KNOW you don't. I'm just saying, if you want them to buy, read and learn from your BOOK, per se, well...

As far as real-life situations go, I'd probably just complain to management and let THEM handle it.