I don't think parents should be allowed punch holes in their kids bodies, no.
And a child who gets Hep. B from a piercing gun has been endangered.
Well, that's your opinion, and thankfully, not the law.
What if your eight year old, or even seventeen year old, wanted to get it done, and you said ok, and she still got Hep. B? You consented, you're still responsible, aren't you? They are still minors.
BTW, there is such a thing as a Hep. B vaccine. I have gotten it, and so have my children, thanks to overseas deployments.