Let me clarify what I meant. The child doesn't need to be able to WRITE out his own narrations, but he should be able to DO a narration. If the ability to retell stories back isn't very good, then I would wait because that is what Writing Tales is all about-retelling the story. They don't have to be able to write it themselves. BUT....It makes a lot more work for the parent if your child can't write it out themselves.
My ds is in 3rd grade, and by this age, he needs to be working on writing because it is developing his hand motor skills. I also let ds type his final product on the computer. I think his spelling and punctuation (including when to use quotations) has improved tremendously because he is doing this himself.
So, all that to say, yes, you can do WT if your child needs help writing, but I think your child will get a LOT more out of it if he can do the writing himself.
JMO