If the kid is smart enough to understand that, he can also reason as follows: I can tell that you really don't want me to swear anyway, regardless of that. And if I'm trading off frequency for impact, it's awfully hard for you to know the exact amount of swearing that optimizes impact. Both of these factors make it especially likely that what you're telling me is motivated reasoning, so I should ignore you.

Again, a kid wouldn't phrase it that way, but he might say something like "that isn't really why you want me to stop!" and basically mean that.

He may even be right. Unless you actually want the kid to swear to a degree that maximizes impact, rather than less than that.