Change your query code to look like this:
@SELECT 'Make a Selection' AS pagetitle, '-' AS id UNION SELECT pagetitle,id FROM modx_site_content WHERE parent=20
Also, I recommend changing the hardcoded [tt]modx_[/tt] to [tt][+PREFIX+][/tt] like so:
@SELECT 'Make a Selection' AS pagetitle, '-' AS id UNION SELECT pagetitle,id FROM [+PREFIX+]site_content WHERE parent=20
That way, you can change your prefix in the future without trouble
You also might want to update your TV’s "Default Value" to "[tt]-[/tt]" (without the quotes) so your TV Content Values table doesn’t get cluttered.
Enjoy!