I don’t think this is only a kids issue. A lot of adults need this too. The addictive apps are very well designed, while most blockers are either too easy to ignore or too annoying to keep using. I built a small iOS blocker because I had the same problem. Making it strict enough to actually work without making people hate it is the main challenge.
As an adult, who despises all those apps, I don't want to grant government the power to make that decision for me.
These giant companies pour millions upon millions of dollars into engineering their services to be as "engaging" (read: addictive) as possible with the specific goal of making users spend more time on them.
Against that, the average person has no chance. The power balance is hugely uneven.
A responsible government which actually cares for its people has a duty to protect them from abuse like that.