I agree with this and generally think parents need to do better and be more responsible for their kids' moral, social, and emotional development. But, here we are where porn is only getting increasingly graphic, realistic (consider that a MS student can now make porn featuring his latest crush), and accessible. Parents, especially those with limited resources, cannot keep up and don't know what they don't know. They just know Johnny wants a phone and that's good for me because I can keep track of and communicate with him while I'm at work.
My oldest is 6. We use the Amazon Fire tablets you mentioned. They still get into stuff we don't really like and, from what I can tell, we can't easily control it, but it generally works.
I don't really know what we'll do as they get older. My wife and I both work but we have the luxury of knowing the concerns and knowing where to go to attempt to address them. We have ongoing and developmentally appropriate talks about bodies, privacy, and choice that will continue into more difficult discussions. However, we can't control what they are exposed to at school or, to an extent, at friends' houses. That's a bit worrisome, but we'll have to rely on open communication, general family cohesion, and intentional conversations around these topics to allow us to guide them as best as we know how.
But that's not even my main concern- it's the vast majority of parents who are just unaware, unconcerned, overwhelmed, or uncomfortable with this topic and that doesn't seem to be getting better. So, just like we restrict access for children in all kinds of ways, we should do the same with porn. We of course won't stop it all, but to at least tone down the ubiquity of it, help parents who are trying but have limited means, and knowledge, etc. Maybe that means every company has to offer affordable, child versions of their devices that can't connect to porn websites and can't download full versions of social media apps. I don't know. Again, they're always going to be able to access it, but putting more barriers in the way when there's currently none, would help and would likely raise the age of first exposure, reduce unintentional exposure, and lower the heat in terms of the amount, intensity, and accessibility of this content. With all the technology we have, I'm sure there are other ways that would help create a floor for parents.