I do know that in the united states, a minor owning anything related to adults (sex toys, porn, etc) is illegal and could lead to said adult losing their child and their parenting rights

I highly doubt that is true in the US, in the terms you describe. I doubt that possession of sex toys by minors is illegal, I doubt that it would reflect onto the parents and, even if both of those things are true, I doubt that the sanction would be removal of a child. Citations and examples or it didn't happen. Obviously, such issues might be part of wider concerns if there are welfare issues through to the attention of social services, but crimes? As in "the police come around and arrest you?" Nah: unless you have a case you can point to of the police arresting a 17 year old for possession of a vibrator, I'll assume you're wrong.

In the UK, at least, none of the above is true. There are no legal restrictions in general of possession of "adult" things (there may be restrictions on sale, but those are legal problems for the vendor, not the purchaser), there are no legal implications for parents for the actions of their children, and the decision to remove children from their parents has an incredibly high bar and is a decision taken by courts and social services purely as a means of protecting children from harm, not punishing parents.

And someone who says they live in Knightsbridge is either lying about their location or extremely rich, and in both cases probably beyond the reach of social services anyway.

"I will be going through my child's mail until the day they are 18 to make sure that they don't have anything inappropriate for their age" would be the sign of a controlling, abusive parent unable to establish healthy boundaries with their child, so you're no doing yourself any favours here.