Obviously sympathetic to the mothers position here. However, based on my limited understanding of how this stuff works, if content is being viewed on 'the dark web' it likely wouldn't be obvious to internet providers, nor would a kid who's savvy enough to be on the 'the dark web' in the first place be really prevented from finding a work around to any potential block in place.
I'm a parent. This stuff scares the crap out of me, but I'm not sure we can actually legislate this stuff out of existence.
It's not a secret browser, it's the system browser. Most apps do not have a browser to view web pages, they use the system one (eg to display terms and conditions pages). The exploit here involved someone accessing such a page and sidestepping into another, because the page had links away, allowing a Google search or something and avoiding restrictions that were only applied to the main browser app.
This is more of a failure in parental control features not being fully comprehensive. The story is also much older than a couple days.