I think both AVNicholls and Patryn explain this fairly well. But if you still find it hard to explain this to your JTE, this would be my approach (Disclaimer
: not a grammar nerd so I can't say I'm correct in my explanation).
First, for a simple explanation of "was" vs "had been": http://english.stackexchange.com/questi ... i-had-been
Using the change of state definition, I would explain it by trying to convert everything to this form:
When she <time frame>.
So breaking down the first example you gave, you'd get:
"has been" implies a change of state => She had been a child. => When she stopped being a child. => "stopped being a child" is the time frame being defined.
"since" also implies a change of state => Since when did she want to be a writer? => Since she was a child. => When she was a child. => "was a child" is the time frame being defined.
Combining the two ("since" and "had been") is not a commonly accepted form of usage because it create ambiguity in determining the proper time frame. To explain, this is what would happen using the above format:
Since when did she want to be a writer? => Since she had been a child. => When she had been a child
. => When when she stopped being a child
This makes absolutely no sense because "when" is not referring to a proper time frame. Additionally, as both AVNicholls and Patryn pointed out, "had been" and "was" do not mean the same thing. So even if the usage was correct, the two sentences are not interchangeable as you'd be changing the meaning of the sentence.
Not sure if this explanation makes sense or is even correct, but it's how I would make sense of the two.