I'm not entirely sure that I understand fully but I think you are asking this.
Open a new child form from the MdiParent:
Code:
Public Class Form1
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim f As New Form2()
f.MdiParent = Me
f.Show()
End Sub
End Class
Open a form from that child that is not a child form and then hide everything else:
Code:
Public Class Form2
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dim f As New Form3()
f.Show()
Me.Parent.Parent.Hide()
End Sub
End Class
Hope that helps a little...if I got the question right.