What is the proper way to include required php in child theme?

I’ve created a child theme and everything is working fine.

However, I want to break out some of the php in functions.php into another file in my child theme includes folder.

If I add this to functions.php it throws an error:
require( get_stylesheet_directory() . '/includes/ajax.php' );

Warning: require() [function.require]: URL file-access is disabled in the server configuration

If I move the php to the parent theme includes folder it works.

require( get_template_directory() . '/includes/ajax.php' );

I want my child php to stay with my child theme folder. What is the proper way to include my child theme php? I don’t want to change php.ini to adjust the security settings.