Teddy bear - how do you propose to stop your children being shown porn on other kids' phones? Vet their friends? How big a friendship pool do you think they'll have?
How do you propose to stop them being shown porn by other kids at school? Home-school?
Yes, restrictions on access help (whether that's giving them a Nokia brick, no phones in bedrooms, blocks on the router). But they can never be the whole solution, partly because these days kids need access to tech for school (and then it becomes a question of how early they learn about VPNs), and partly because at some stage 11, 14, 16, - you will need to take a step back as parent and allow them to regulate their own behaviour.
That's why there's no technological magic bullet. Use tech fixes to minimise risk, yes, but you have to talk to them too.
And keep putting it in the context of kindness and respect for other people. Does what you're seeing on screen look kind? Does it look respectful? If not, it is a bad form of sex.
And don't shy away from moral judgements - but make sure you can back those judgements up with arguments. Teens hate being preached at. What you're aiming for is opening them to the Idea that it's possible to think critically about whether sex acts are good or bad for the people involved, rather than having them mindlessly bleat the so-open-minded-their-brains-have-fallen-out mantra of "your kink is not my kink and that's okay."