What if you did something like this:

In the Main MDI Form or a module create a public sub like this:

Public Sub UnloadForm(ByRef MyForm As Form)
Unload MyForm
End Sub

Then from the child form you are trying to unload do this:

Private Sub Form_Unload(Cancel As Integer)
MDIForm1.UnloadForm Form1
or
subMain.UnloadForm Form1
End Sub