class MdiChild : Form{
     // ... 
  }

  class MdiChildManager {
    private static MdiChild child = null;
    public static Form Create(Form mdiContainer)
    {
      if(child != null)
        child.Close();
      child = new MdiChild();
      child.MdiParent = mdiContainer;
      return child;
    }
  }

  class MdiContainer : Form{

   // ... 

  private void OnMenu_NewChild(...){
    Form child = MdiChildManager.Create(this);
    child.Show();
  }

   // ... 

  }