What a horrible reaction to confront the kids to such an extent that they end up crying their eyes out, or feel the need to lie about it is terrible. I'm guessing in the years leading up to giving them tablets their parents did not show them how to properly use the internet, and spend time with them as they used the internet to build up trust in them and instill in them the sense to make good choices when they stumble across, or seek out whatever might interest them or be curious about.
I'll never forget how my mom reacted when after my friend and I (we were 11 at the time) found someone's secret stash of porn mags in the woods, and after rushing home with our ill gotten booty, we were quietly looking at the magazines in the living room back at my house. My mom walked by the living room and saw us huddled around these magazines and wondered what had so captured our attention... so she walked over and saw that we were looking at a Playboy and simply said, "oh... well she's pretty, and I like her smile." And that was it and carried on with the rest of her day... and never mentioned the magazines again. But then she knew and trusted that we were basically good kids, so why freak out and cause a scene and be made to feel ashamed over natural curiosity.
Sure... these days there is a whole lot worse on the internet than a simple Playboy, but that's why you spend time with them in the early years and instill in them the ability to make good choices among other things like not to be a troll and all that, and then trust in them that they will. You can't shield them from all the nasty stuff on the internet, because there is plenty of nasty stuff even on sites like twitter and fb.