The rest of the questions seem valid though.
So no, the onus of evidence is on people claiming that rough language hurts children in some way. (Note we're not talking about abusive language here.)
> but, I don't think this is really what people want their kids reading […]
This is actually the initial claim, and the Vonneguts quote was a response to this claim.
Parents used not to care about what their children hear or read, until society told them it's bad parenting.
> don't damage children much
Not every statement should require extended proofs. It's probably the case that you can't think of any children being hurt by coarse language, nor of having heard of any reporting of this. Absence of evidence is evidence of absence.