type
  IParent = interface
    ['{........}']
    procedure Proc;
  end;

  IChild = interface(IParent)
    ['{........}']
  end;

  TParentImpl = class(TInterfacedObject, IParent)
    procedure Proc;
  end;

  TChild1Impl = class(TParentImpl, IChild)
    procedure Proc;
  end;

  TChild2Impl = class(TParentImpl, IParent, IChild)
    procedure Proc;
  end;

procedure TParentImpl.Proc;
begin
  ShowMessage('Parent');
end;

procedure TChild1Impl.Proc;
begin
  ShowMessage('Child1');
end;

procedure TChild2Impl.Proc;
begin
  ShowMessage('Child2');
end;

var
  LParent: IParent;
  LChild: IChild;
begin
  LChild := TChild1Impl.Create;
  LChild.Proc;  // Child1
  LParent := LChild;
  LParent.Proc;  // Child1
  LChild.QueryInterface(IParent, LParent);
  LParent.Proc;  // Parent

  LChild := TChild2Impl.Create;
  LChild.Proc;  // Child2
  LParent := LChild;
  LParent.Proc;  // Child2
  LChild.QueryInterface(IParent, LParent);
  LParent.Proc;  // Child2

  LParent := TChild1Impl.Create;
  LParent.Proc;  // Parent

  LParent := TChild2Impl.Create;
  LParent.Proc;  // Child2
end;