Well, I thought it was so obviously “self-cockblocking” that it would be apparent there was no “super secret hidden extra curse word”. Given that, I didn’t think it would come across as patronizing, infantalizing, or as gaslighting, but instead as just a bit silly. Since you know, there really is no other word that makes sense, so the “lie” would be abundantly apparent and as such no insult could be taken.

To add on to that, “we will tell you when you’re older” is just what people say when they don’t want to explain an obscene word or concept, which fit well with the idea that this fictional word was “unfit for polite discussion”.

So with all that in mind. Since OP rightly pointed out that “cock” really isn’t worth censoring, I felt it would play on the readers expectations that, if cock wasn’t worth it, there must be some worse hidden word that justified the astrix!

And we could all laugh,
“oh ho ho! They are using the super secret hidden curse word that must never be spoken!”
we’d say.

Comfortable in the knowledge that in reality it was just a bit of overly cautious censoring of cock… And we would all have a good chuckle.

I’m really not great at jokes and I’m sorry if this one made anyone feel patronized, gaslit, or infatalized.

Edit: if I had a do over, I’d say:

Oh, that’s not cock… Nobody would censor cock… That must be! gasp… No, nevermind, I dare not say it, it is forbidden.