Is post category(ies) a child of parent category?

I’ve been struggling to figure out how to do this. Seems like such a simple task. I have a hierarchy of categories X levels deep, and I want to render recent posts from any category that is child of a more general category… and here’s the function to get things started. This function will take a post ID, and the parent category you want to see if it is a child of…

(This way it is more dynamic than hardcoding (is_category(1) || is_category(2) etc. etc.)

function post_child_of_cat($child_post_id, $parent_cat_id)
	{
		$cat_array = wp_get_post_categories($child_post_id);
		foreach ($cat_array as $category)
		{
			if (cat_is_ancestor_of((int)$parent_cat_id, (int)$category))
			{
				return true;
			}
		}
		return false;
	}

So, I have a category like so

Fruits > Green > Apples

And I want output recent posts from any category under fruits on any of the apples pages… lets assume the fruit category has a category id of 69. I want to put this code in the template that the posts use, inside the loop,

for example, put this in the loop in single.php..

if (post_child_of_cat($post->ID, 69)) {
		// do whatever you want here...

	}