I really wouldn’t write off the Shield completely. It’s a few years old, but it works really well. My TVs are all disconnected from my network, and each has a Shield attached. The Shield can stream 4k HDR from Jellyfin, play ad-free YouTube with SmartTubeNext, and handles remote game streaming at 4k/60 with Sunshine/Moonlight. It’s really a versatile little box.
Social media companies, adult websites, whatever, can try to find ways to block children from accessing their content, but kids will always find a way around it.
It’s the parents’ responsibility to control their children. I’ve said 1000 times, children don’t need access to smartphones and tablets. A desktop PC or laptop with strict parental controls is adequate enough for school work, learning about technology, and some basic entertainment.
When a child is old enough to work and pay for a smartphone themselves, then they’re old enough to have a smartphone. A prepaid flip phone with basic voice and SMS is more than enough for a 15-year-old.