var h:longint;
   s:string;
begin
h:=FindWindowEx(handle,0,'TEdit',nil);
h:=FindWindowEx(handle,h,'TEdit',nil);
s:='Text';
sendmessage(h,wm_settext,0,longint(s));

...
 sStrClass, sWindowName, sStrToSet: String;
...

function EnumProc (W: HWnd; Param: LongInt): Boolean; stdcall;
var
 WName: array[0..255] of Char;
 CName: array[0..255] of Char;
begin
 GetWindowText(W, WName, 256);
 GetClassName(W, CName, 256);

 if (AnsiCompareText(sStrClass, CName) = 0) and (AnsiCompareText(sWindowName, Nm) = 0) then
   SendMessage(W, WM_Settext, 0, Integer(PChar(sStrToSet)))
 else
   begin
     EnumChildWindows (W,@EnumProc, 0);
     EnumProc := True;
   end;
end;

procedure FindAndSetText(ParentHnd: HWND; const StrClass, WindowName, StrToSet: String);
begin
 sStrClass := StrClass;
 sWindowName := WindowName;
 sStrToSet := StrToSet;
 EnumWindows(@EnumProc, 0);
end;

FindAndSetText(Form1.Handle, 'TEdit', 'Edit1', 'My Text');

EnumWindows(@EnumProc, 0);