Y'know what happens instead? Their friends have unfettered access to smartphones, so my nephews see all the idiotic brainrotting shit on youtube shorts anyways. If not at home, they'll see all the crap we're blocking at school from their friends anyways. Hell, they could just go to the library and access the free public computers there if they wanted to! So my sisters who are responsible and do everything correctly, still suffer, and like any reasonable parent don't want to go to the extreme of locking their children up in cages and not letting them outside of the house.
There's a reason we don't legislate alcohol and tobacco sales to the same tune of "Just police your kids 24/7 and keep them under lock and key", we instead realize that we can't (or, shouldn't) supervise kids 24/7 day in and day out, and have society-wide rules that forces everyone to not sell booze to anyone underage.