Data sharing between childforms, plz help

I was wondering if anyone could help me with this...
I am having problems sharing data between child forms.
the setup is as follows:
mainFrm <--- MDIForm
|----> ViewFrm <----ChildForm
|----> SystemFrm <---ChildForm

dim Data() as CustomClass
Public GetData() as CustomClass
public SetData(x, data)

Now when I load ViewForm I can use 'data' to fill a tree control. However
the problem starts when I want to load SystemForm after ViewForm. Now the
tree in systemform cannot be loaded anymore.
Likewise when i start with systemform first the tree can be filled but then
the tree in viewform cannot.
Also if i switch back to the first form the tree is empty and will no longer
be able to fill the tree.

I have tried to unload the form before loading in the new one but that does
not do anything.

Please, if anyone could help me with this...im pretty stuck here...

thanks,

Paul Schouten