To get list of bottom most or deepest or last child for specified parent category

You can use wp_list_categories to achieve that:

<ul>
    <?php
        wp_list_categories( array(
            'child_of' => <PARENT_ID>, // show only children of PARENT_ID
            'childless' => true, // show only categories without children
            'hide_empty' => true, // should empty categories be hidden
        ) );
    ?> 
</ul>

You can find full list of available params here:
https://developer.wordpress.org/reference/classes/wp_term_query/__construct/#parameters

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