Purely in terms of freedom -- without arguing for any other pros and cons -- is this worse than mandatory school attendance, or not being able to vote, or take medical decisions for themselves, or whatever else kids cannot do these days.
And much like with this new Chinese law, many minors violate all of those restrictions often with parental approval.
And parents are constantly adjusting the things their kids can or can't do, often requiring kids to ask permission to do most things. Its a way of protecting them from their immature decision making ability (with functional parents anyway).
So I think the aversion is more based on the surprise that it seems to go 'further' than we'd expect in the west, and politics/culture turns that into something to criticize the CPC (or whatever governing body responsible) for because COMMUNISTS!!!
I don't think this is a case of "because..... communists". This move is rightly criticized in my opinion.
But this is just a declaration, not an argument (there's nothing wrong with that.) Should parents be able to let their children smoke or do porn?
Do you need any more than that? From the western point of view this is the apex reason.
EDIT: To add - it is true in the west that minors don't enjoy all the same freedoms as adults. However for most things those decisions are made by the parents and not the state.