Try This !

Sub sUnloadChild()
Dim intIndex As Integer
Dim blnLoop As Boolean

blnLoop = True
Do While blnLoop
'Forms is a collection of loaded form in your App
'Index 0 is MDIfrom
For intIndex = 1 To Forms.Count - 1
If Forms(intIndex).MDIChild = True Then
Unload Forms(intIndex)
blnLoop = True
Exit For
End If
blnLoop = False
Next
'Only MDI from is open
If Forms.Count = 1 Then blnLoop = False
Loop

End Sub