Hi Philippe, there is one crucial condition for opening a light box on the parent frame: both parent and child frame must live on the same domain. If not, the script will be blocked by any browser that respects cross-site scripting security.
Next, it’s important to realize that this is not possible with the standard WordPress plugin. It would either need a heavy modification or (better) a custom implementation of the light box script on the child page.
I’m afraid this is out of the scope of possibilities of Easy FancyBox…