That's 14-15 years old in these parts. Old enough to marry in many states. Hardly a "child" in the sense you suggest

In what sense is that?
A girl of 14-15, here 8th or 9th grade can be 13-14.
The suggestion you make is that 'heaps' of girls that age are all 'sexually' mature individuals, when the reality is that they aren't. They are still children in the sense that the majority are in SCHOOL, they live at home and I would doubt their 'first' goal would be to be 'married' in all that this entails ie they are minors.

but I'm not going to even continue in this train because I know it's gonna upset many, but it's food for thought either way.

I'd be really interested as to which writer would feel 'comfortable' ultimately to write the story using a girl of that age.

And if it's a writer that is considering a 'professional' career, then perhaps a little more food for thought.. because ultimately karma does exist and literary things usually do come back thricefold.