> I will absolutely be barring my children from [drugs]. I fully expect them to use it or similar sorts of stuff behind my back, but that's okay. It will then be hidden and scarce, which limits the overall negative consequences it can have.

If you make a simple substitution, it becomes clear that "limits the overall negative consequences it can have" in no way logically follows from "behind my back" and "it will then be hidden". Draconian bans are, simply put, extremely lazy parenting, and not only lazy, but ineffectual. All the more so when it's something as relatively innocuous as social media, where your children will be actively shunned by their peers and come to resent you while still finding ways to use it. If you want good outcomes for your children, play an active role in their life and guide them positively instead of thinking you can just say "don't do X" and that will magically be the end of all problems.