mdiparent y mdichildren

[xknown]

Ubicas el código en tu formulario puesto como MDI:

Código:

Public instancias As New Hashtable()

Public Function AbrirVentana(ByVal tipo As Type) As Form
    Return AbrirVentana(tipo.FullName)
End Function

Public Function AbrirVentana(ByVal tipo As String) As Form

    Dim formulario As Form = CType(instancias(tipo), Form)
    If formulario Is Nothing OrElse formulario.IsDisposed Then

        formulario = CType(Activator.CreateInstance(Nothing, tipo).Unwrap(), Form)
        instancias(tipo) = formulario
    End If

    If Me.IsMdiContainer Then
        formulario.MdiParent = Me
    End If

    formulario.Activate()
    formulario.WindowState = FormWindowState.Normal
    formulario.Show()
    Return formulario
End Function

Saludos