How to get child theme templates when loading theme via add_filter(‘template’)

I’m trying to load a child theme when on certain posts, pages or categories with the code below. It loads the child theme’s css just fine, however, the child themes’ template files don’t load.

If I explicitly set the child theme via the Appearance setting in WP 2.7 everything works as expected. What am I doing wrong or not doing that I should do?

function mp_get_theme() {

    global $wp_query;
    $post = $wp_query->post;
    $id = $post->ID;

	// should I load another theme?

	if ($post->ID == 836 || $post->post_parent == 836 || (is_archive() && is_category('14')) || (is_single() && in_category('14')))
	{
	return 'newtheme';
	}else{
	return 'defaulttheme';
	}
}

add_filter('template', 'mp_get_theme');
add_filter('stylesheet', 'mp_get_theme');