How to pass a handle of a MDI child form

this code returns an exception (EAccessViolation) when i call the Method
"Add" of
the RichEdit control in a MDIChild window

procedure AddRoot(StringBuff : String; NodeFields : TDataset ;RtfObject :
TForm;UserPref : TStringList);
begin
...
(RtfObject as TMDIChildRTF).RichEdit1.Lines.Add(#10 + #10);
...
end;

this one returns the same

procedure AddRoot(StringBuff : String; NodeFields : TDataset ; RtfObject :
TMDIChildRTF ;UserPref : TStringList);
begin
...
RtfObject.RichEdit1.Lines.Add(#10 + #10);
...
end;

I just call this procedure afther creating the TMDIChildRTF instance passed
to the procedure.
Is there a solution that let me use a pointer to reach a ChildForm like
this way ?

p.s.. sorry for my bad english...