Generally speaking, kids find pornography websites on the Internet the same way adults do…they look for them. Porn websites are actually hard to accidentally trip over nowadays thanks to search engine filtering that no longer allows them to pass themselves off as something else in the search results. 
Sure, a non-porn site could have given up its old domain name for a newer, better one, and had the old one bought up by a porn site. That definitely does happen. But the simplest answer is generally the right one…the kid didn’t accidentally go to the porn site.
Granted, should a kid go to a somewhat shady site, such as a warez site…where people go to try to download software illegally to avoid paying for it…he will very likely come across porn sites, links and banner ads because that’s how many of those site owners make their money. Being curious, he may very well click on that link or ad.
And, having done so, even if he closes the window, he may be faced with another porn site after another after another. Yes, even with a pop-up blocker.
