try this :
type
  TEnumProc = function EnumChildProc(inwm:hwnd;wparam:integer):boolean of object cdecl;
  TwinFind = class(TObject)
   ...
  private
    EnumChildProc : TEnumProc;
   ...

procedure TwinFind.getWindowData(windowName:string;Sender:TObject);
begin
  //
main_win:=FindWindow(nil,lpctstr(windowName));
if main_win = 0 then Exit;

temp_win:=0;
while EnumChildWindows(main_win,EnumChildProc,0)<>false do
begin
  //
end;

or this :

try this :
type
  TEnumProc = function EnumChildProc(inwm:hwnd;wparam:integer):boolean; cdecl;
  TwinFind = class(TObject)
   ...
  private
    EnumChildProc : TEnumProc;
   ...

procedure TwinFind.getWindowData(windowName:string;Sender:TObject);
begin
  //
main_win:=FindWindow(nil,lpctstr(windowName));
if main_win = 0 then Exit;

temp_win:=0;
while EnumChildWindows(main_win,EnumChildProc,0)<>false do
begin
  //
end;