gistfile1.php

function get_deep_child_category()
{
$categories = get_the_category();
    $maxId = 0;
    $maxKey = 0;
    foreach ( $categories as $key => $value )
    {
        if ( $value->parent > $maxId )
        {
            $maxId = $value->term_id;
            $maxKey = $key;
        }
    }
    
    
    
    return  $categories[$maxKey]->term_id;
}