Post the XML of your window.
matty's reply to Cannot CreateChildWnd
xml code:
<!-- The child window -->
<Window Id="WndChildExample" Version="1">
<ChildTmpl/>
<Position Width="243" Height="193"/>
<Controls>
<Control xsi:type="StaticControl" Id="lblTitle">
<Position Top="25" Width="100" Left="15" Height="15" />
<Caption>hi I am a child window :)</Caption>
</Control>
</Controls>
</Window>