Doesn’t change my opinion on fictional characters that are made to look like children when making them look older has absolutely no effect on the story.

In anime people also care about visuals, not only the story. Making certain characters have youthful looks is done to appeal to people who have preference for youthful looks, sometimes including sexual preference, and you also seem to agree it’s absolutely legitimate preference to have in case it’s limited to actual adults. The visual difference between “looking like children” and “having youthful looks” can be quite a subjective one, and that’s where everyone draws the line for themselves.