Actually I see on the child select page you have the select list results wrapped in a Show if Recordset is Empty server behavior.
That would ensure that no results are ever displayed from the child list. I'd delete lines 32 and 21 to see if that helps, but that wouldn't cause a sql error.
I also see your div tags aren't aligned properly on the main page. You have two open <div> tags inside your <select> before the </select>... that I'm sure would cause problems too.
Move the </select> tag on line 52 up to after line 49 so the divs that follow aren't contained in the select.