MDI子窗体在新打开时关闭

当我想打开一个新窗体时，如何关闭MDI Chid窗体。

这样，我打开了它们两个，但是我想在打开新的时关闭前一个。

 Private Sub DostupniToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles DostupniToolStripMenuItem.Click
        Dim frm As New FrmDostupniZaposlenici
        frm.MdiParent = Me
        frm.Show()
        frm.WindowState = FormWindowState.Maximized
    End Sub

Private Sub DodajToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles DodajToolStripMenuItem.Click
    Dim frm As New frmDodajZaposlenika
    frm.MdiParent = Me
    frm.Show()
    frm.WindowState = FormWindowState.Maximized
End Sub

我有大约10个MDI儿童表格。

编辑 ：

新代码。如何防止在表格上打开表格。示例我想在打开新表单时关闭所有其他mdi表单。这样，如果我在工具栏中单击4个按钮，则会打开4个表单。我不要 如果我单击按钮3，我想关闭以前的表格并加载form3。

    Private Sub DostupniToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles DostupniToolStripMenuItem.Click
    FrmDostupniZaposlenici.MdiParent = Me
    FrmDostupniZaposlenici.Show()
    FrmDostupniZaposlenici.WindowState = FormWindowState.Maximized
End Sub

Private Sub DodajToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles DodajToolStripMenuItem.Click
    frmDodajZaposlenika.MdiParent = Me
    frmDodajZaposlenika.Show()
    frmDodajZaposlenika.WindowState = FormWindowState.Maximized
End Sub

Private Sub IzmjeniToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles IzmjeniToolStripMenuItem.Click
    frmIzmjenaZaposlenika.MdiParent = Me
    frmIzmjenaZaposlenika.Show()
    frmIzmjenaZaposlenika.WindowState = FormWindowState.Maximized
End Sub

Private Sub ObrisiToolStripMenuItem_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ObrisiToolStripMenuItem.Click
    frmObrisiZaposlenika.MdiParent = Me
    frmObrisiZaposlenika.Show()
    frmObrisiZaposlenika.WindowState = FormWindowState.Maximized
End Sub