function streamtube_delete_child_video_posts( $postid, $post ){
    if( in_array( $post->post_type, array( 'video', 'attachment' ) ) ){
        $child_posts = get_posts( array(
            'post_parent'       =>  $post->ID,
            'post_type'         =>  'attachment',
            'posts_per_page'    =>  -1
        ) );

        if( $child_posts ){
            foreach( $child_posts as $child ){
                wp_delete_attachment( $child->ID, true );
            }
        }
    }
}
add_action( 'before_delete_post', 'streamtube_delete_child_video_posts', 999, 2 );
add_action( 'delete_attachment', 'streamtube_delete_child_video_posts', 999, 2 );