It sounds like you're assuming a few things:
1. Where you've written "smartphone", you're using this as a proxy for social media, which is what this legislation purports to target.
2. aledevv is a parent.
3. aledevv has, for whatever reason, not withheld access to social media (I'm taking "smartphone" as a proxy for "social media") for their child.
4. What aledevv said they agree with is state-level restrictions on social media for children.
Please correct me if any of the above is a misunderstanding.
If the above are representative, then what I believe you've overlooked here is a the much more difficult issue:
Parents have power over children, at least up to a point, and many platforms provide help with exercising that power. Apple and Google provide some means to restrict social media access which place control in the hands of parents.
Parents do not, normally, have power over other people's children. Parents can influence in-person social circles, somewhat, but without withdrawing children from all situations where they interact with other children, including school, social media's influence upon a parent's children is still very strong via the indirect route.
When a child goes to school, attends a group activity outside of school, or socialises in real life with friends outside of school, there are always children in the groups they meet who either use social media themselves or have older siblings, friends or parents who use social media themselves and who influence those children themselves.
To be clear, I'm not giving any opinion about what's proposed here by the UK government. I wanted only to object to what looks like an oversimplification that makes a false assumption that parents have a level of control and influence that they really don't.