So nice of you to know what I want without me even implying it

Yet you did imply it, as I said, by mentioning the age of the persons involved.

There is no accurate way to know age, without some form of identity or age verification. Presuming a child will have an account marked "child" is folly, for kids can just sign up without a parent's knowledge, creating a second account. If the goal is to actually protect and be a pseudo parent for the child, then actually ensuring that a child cannot have an adult account is part of that.

My point is, TSA style "we're doing things which look secure, but are not helpful and only inconvenient" isn't going to help. It will only give the appearance, not the actualized result of security.

No, the child can mark itself as a child. The implication is in your imagination. There are lots of children (even most) which do not feel the need to upage themselves on the internet, but where they get only the downsides of children accounts and get utilized by the platform.