Here is how it can be done. This takes into account all docked controls (including menus) in the mdi parent form.

[C#]

private void FillActiveChildFormToClient() 
{ 
  Rectangle mdiClientArea = Rectangle.Empty; 
  foreach ( Control c in  Controls ) 
    if (c is MdiClient ) mdiClientArea = c.ClientRectangle; 

  ActiveMdiChild.Bounds = mdiClientArea; 
}

[Visual Basic]

Private Sub FillActiveChildFormToClient()
  Dim mdiClientArea As Rectangle = Rectangle.Empty
  Dim c As Control
  For Each c In Controls
    If TypeOf c Is MdiClient Then
      mdiClientArea = c.ClientRectangle
    End If
  Next
  ActiveMdiChild.Bounds = mdiClientArea
End Sub 

Contributed from George Shepherd's Windows Forms FAQ