加一个命令按钮，按钮的单击事件加入如下代码即可

Dim i
For i = 0 To Me.Controls.Count - 1
 If Me.Controls(i).ControlType = acSubform Then
  DoCmd.OpenForm Me.Controls(i).Name, acPreview
  Exit For '只打印一个子窗体，如要打印多个子窗体可去掉该句或注解
 End If
Next i