Need to style all child, grandchild, etc pages

I’ve tried using the following conditional statement to add a banner to all children of certain pages:

elseif ( is_tree(array('6', '10', '11', '21', '22', '44', '203', '382', '508'))) {
    	// the page is "meditation", or the page is in the "guides" section and so if a child of that page
    	echo '<img src="/images/breadcrumb-guides.png" />';

That adds the banner to the children of the specified pages, but not to the children’s children. How would I get the styling to apply all the way down the hierarchy? don’t think I have any nesting deeper than the grandchild level.