You can disable the opening Form instead of making the modal child truly modal..
You can try this: Open the ‘modal’ children with
this.Enabled = false;
FormDlg yourModalChildForm= new FormDlg(this);
yourModalChildForm.Show();
In the constructor write :
    Form myParent;
    public FormDlg(Form myParent_)
    {
        InitializeComponent();
        myParent = myParent_;
    }
And in the FormClosed write:
  private void FormDlg_FormClosed(object sender, FormClosedEventArgs e)
  {
      myParent.Enabled = true;
  }
solved Is it possible to create a dialog which only blocks its parent? [closed]