If in_category?

You’ll need to create a custom function for this:

https://codex.www.ads-software.com/Function_Reference/in_category#Testing_if_a_post_is_in_a_descendant_category

Not working for me ??

In functions.php i have inlude this code:

<?php if ( ! function_exists( 'post_is_in_descendant_category' ) ) {
	function post_is_in_descendant_category( $cats, $_post = null ) {
		foreach ( (array) $cats as $cat ) {
			// get_term_children() accepts integer ID only
			$descendants = get_term_children( (int) $cat, 'category' );
			if ( $descendants && in_category( $descendants, $_post ) )
				return true;
		}
		return false;
	}
}
?>

and in my theme file:

<?php
$cat = get_the_category();
if ( post_is_in_descendant_category(7) || is_category(7) || in_category(7) || cat_is_ancestor_of(7, $cat) ) { 

} ?>

Any idea?

Try post_is_in_descendant_category( 7, $post )

No.. Nothing ??