I have 2005, and don't see how to create an MDI app, though I have in the past one so with VB6.
I feel pretty confident that there is a collection of windows, perhaps as part of the Application or App object. Each of the elements in that collection may have an .activated or .focus or .HasFocus property. I'd look around for something like that.
Also I suspect that in the creation of a new child, the functionality has a way to receive a handle to that window at the same time. Perhaps creating the child as a variable:
Dim fA_Child As frmOneOfMyForms
fA_Child = New frmOneOfMyForms
or something like that. If you then ran the 2nd and 3rd line a second time, the 1st form would still be open (and you lost your reference to it), and fA_Child is now a reference to a 2nd child.
Having a reference, you then have access to all of the public properties and exposed objects of the form that it is a reference to.
Is that clearer? Perhaps I'll poke around on this a bit at home over the weekend. (I do have a lot other to do...)