Tell them no porn, no clicking on pop up ads, no cheezy viral articles with click bait titles, etc.
If you can visit them in person, check their computer for malware.
Find out what they want to use the internet for and help them create a white list. If they have enough safe ways to satisfy their needs, there is less temptation to randomly venture forth into things they don't really understand.
If you are able to talk about porn and ban it with your parents (presumably providing some alterate means, because you know they have needs), then well congratulations that sounds like a very open and sharing relationship.
I would just make a written checklist of hard "no" items and list porn on it. If they call with a problem, ask "Were you clicking on a hard no item?" and don't get into whether or not it was porn.
If they call frequently because their computer is a giant mess of malware, eventually tell them they need to educate themselves because your efforts to keep them safe are proving insufficient.
If you honest to God can find no means to tell your parents that porn is a really big problem with regards to technical safety online, then I suggest you wash your hands of this issue and tell them you are wholly unqualified to help them and maybe point them to some resources to help them sort it out themselves. Hopefully, buried somewhere in those resources is the fact that porn is a problem.
Otherwise you are doing more harm than good by giving them the illusion of assistance when the real message is "Sorry, I can't actually protect you because it might involve admitting my parents, who probably did the wild thing to get me, might still have a sexuality." If that's the answer, don't pretend to help. Just refer them elsewhere to someone willing to have that conversation.
I will add that you need to know porn is an issue even if you don't consume it. Advising them that porn is a problem doesn't actually presume they consume it. I had to do a hard shut down of a laptop because I was moderating a forum and someone posted a porn link. I wanted to do my due diligence and not just assume. It opened a zillion popups and locked my computer up.
Tell them no porn, no clicking on pop up ads, no cheezy viral articles with click bait titles, etc.
If you can visit them in person, check their computer for malware.
Find out what they want to use the internet for and help them create a white list. If they have enough safe ways to satisfy their needs, there is less temptation to randomly venture forth into things they don't really understand.