It doesn't mind that it's being shown, the point is that your nephew shouldn't be able to disable it. So don't give him administrative rights on the Windows PC he uses.
Then probably he'll use a tablet, which makes it necessary to block the site in the router. Some routers might be able to do such.
I have a nephew who has an addiction to internet for cartoons and a site called fan fiction (which has story character called Dragon ballZ ) . Is there any software (paid or free)with which could block it. K -9 protection blocks it . However it shows that that K-9 is being used. Secondly if such a software is used can it controlled by an I phone app.

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