“fictional children can’t consent!!”

Neither can fictional adults!

They are fictional, they have no free will. They have no ability to give or revoke consent. They’re toys, they’re ideas.

“fictional adults can consent if I write them consenting!”

Sure.

And someone else can write a fictional child that can and does give meaningful consent.

That’s not the same? How?

It’s all made up! It’s all imaginary!

Either consent is meaningful in the context of the fiction being written OR all fiction is immoral because you believe it violates a character’s free will.

What’s it gonna be?