imagine if they didn't though because it was just societally accepted that kids having access to sugary-cereal-producing magic rectangles was completely normal, even out in public—plus adults also use their own magic rectangles almost as much as kids, and kids learn by watching adults.

What stops you from making this argument to allow kids to smoke cigs? That's kinda how this is. Imagine if every kid smoked, and if you didn't want your kid to smoke too, they become a social outcast with all the mental issues that come with it.

Society steps in to regulate harmful/predatory behaviors all the time, even between consenting adults (let alone children, even if their parents play along). Libertarianism just doesn't work for kids. (I think it doesn't even work for adults, personally)