This post is old but maybe I can help someone with this issue. I don´t know if this solutions worked for you but I had to do something different. I'm using vb.net though.
I created a Shared Sub in the Mdi Parent Form
Shared Sub CenterFormToScreen()
And then in each of the Mdi Childs
I wrote this code calling the Sub:
With this I'm simply updating the Main Form (Parent) to return to the center of the screen each time one of the children are shown.
If your problem wasn´t this, then don't mind me.