Private Sub frmMain_MdiChildActivate(sender As Object, e As EventArgs) _
      Handles Me.MdiChildActivate
 
        If Me.ActiveMdiChild IsNot Nothing Then
            AddHandler Me.ActiveMdiChild.FormClosed, AddressOf _
              mdiChildClosedHandling
        End If
 
    End Sub

    Private Sub mdiChildClosedHandling(sender As Object, e As _
      System.Windows.Forms.FormClosedEventArgs)
 
        If MdiChildren.Length <= 1 Then
            'Timer starten
        End If
 
    End Sub