See, you've answered a different question. The question you answered is, "why should children be protected from the Internet?" I'll give you for free that children should be protected from the Internet, for the reasons you've said, and now you get to convince me that I, who don't have or plan to have children, should spend my money to protect other people's children from the Internet.
For the same reason your taxes pay for schools even when you don't have kids.
Because we live in a society.
If you struggle to understand why that matters without reference to more direct personal stakes for yourself, just know that without a society, the children will grow up to rape you, kill you, and possibly consume you for your protein content.