Stopdog, this really isn't your style. I am surprised that your therapist suggested it, does she know you at all or has she cracked through to all the warm fluffy stuff instead �� like a Cadbury cream egg!
Here is a link to a woman's website who appears to know a bit about it
Reparenting the Wounded Child
They all seem to talk about a wounded child and how to heal them.
To me I don't think I would allow a therapist to try this as I would be handing over my whole self, adult and child and the idea terrifies me!