Some problem with AccessibleChildren function declare

I want to find a DirectUIhwnd's child control,so I implement the following code:

function AccessibleObjectFromWindow(Ahwnd: HWND;  dwObjectID: DWORD;  const riid: TGUID;  out ppvObject  ): HResult; stdcall; external 'OLEACC.DLL' name 'AccessibleObjectFromWindow';

function AccessibleChildren(  paccContainer: IAccessible;  iChildStart: LONGINT;  cChildren: LONGINT;  out rgvarChildren: PVarArray;  out pcObtained: LONGINT  ): HRESULT;stdcall; external 'OLEACC.DLL' name 'AccessibleChildren';
//////////////////////////////////////////////////////////////////
var
   x,i:integer;
  pa:PVarArray;
  blub: IAccessible;
  l:longint;

begin
    ....
    ....
   if AccessibleObjectFromWindow(FirstDlgHd, 0, IID_IAccessible, blub) = S_OK then
    begin
      x := blub.Get_accChildCount;
      AccessibleChildren(blub, 0, x, pa, L); // this token has problem, the "pa" paramitere result is $9,and the length of "pa" is zero,
     pa[0].Get_accName(0);//this token will raise an error, pa[0] is nil;
  end;

Wether the declare of "AccessibleChildren" is wrong or I use  "PVarArray" wrongly or improperly ?