Make search only search children of a specific page

Hello,
I have a Wiki section on my site, and I want to modify WordPress’s native search to only search pages in this Wiki, or in other words, only children of page #278.
I’ve succeeded in listing children pages of a given page with wp_list_pages and the parameter ‘child_of’, so I’ve been trying for something like that. Two attempts so far:
In the search page…

<?php if ( have_posts(array('child_of'=>278)) ) : ?>
...
<?php while ( have_posts(array('child_of'=>278)) ) : the_post(array('child_of'=>278)) ?>

And alternatively in my functions file…

function SearchWikiPages($query) {
	if ($query->is_search) {
		$query->query_vars['child_of']=278;
	}
	return $query;
}
add_filter('pre_get_posts','SearchWikiPages');

So far with no luck. If anyone knows how I can do this please help. I would not be opposed to using a plugin, or any other creative solution if it gets what I need.
Thanks!