That's actually an interesting question. As far as I can tell, while it is illegal to show minors porn, I don't think parents are on the hook of their children access it. Common sense would argue that a parent can install all the filters they can, but kids are crafty and nothing will stop a teenage boy from taking care of business, though I'm sure some religious state reps/senators would say otherwise. But, to your point, I don't believe parents incur liability from their kids looking up boobs online, especially without their knowledge. Now if a sicko parent is actively showing kids naughty videos, that definitely is a crime.

https://www.hg.org/legal-articles/showing-porn-to-a-minor-is-it-a-crime-48946