class ChildFrm : public WithChild<TopWindow> {
public:
 	ChildFrm() {}
};

class MainFrm : public TopWindow {
public:
	typedef MainFrm CLASSNAME;
	
	Button btn;
	
	MainFrm() {		
		Zoomable().//with Zoomable if you minimize the window, when ChildFrm become mimimize instead maximize
		Maximize();
		
		Add(btn.SetLabel("Open Child").SizePos());
		btn <<= THISBACK(OnBtn);
	}
	
	void OnBtn() {		
		Single<ChildFrm>().Open();
		Single<ChildFrm>().Maximize();
	}
};

GUI_APP_MAIN
{	
	MainFrm().Run();
}