Numeric slug on child post

As you guessed and @Rarst suspected, there’s a pagination check in wp_unique_post_slug() for hierarchical post types:

preg_match( "@^($wp_rewrite->pagination_base)?\d+$@", $slug )

which will match any numeric only slug, optionally preceded by “page”. To get around this you could use the 'wp_unique_post_slug' filter, basically replicating the original code without the pagination check, eg

add_filter( 'wp_unique_post_slug', function ( $slug, $post_ID, $post_status, $post_type, $post_parent, $original_slug ) {
    if ( $slug !== $original_slug && is_post_type_hierarchical( $post_type ) ) {

        global $wpdb, $wp_rewrite;

        $slug = $original_slug; // Undo any previous processing.

        // The following is just a copy & paste of the WP code without the pagination check.
        $feeds = $wp_rewrite->feeds;
        if ( ! is_array( $feeds ) )
            $feeds = array();

        if ( 'nav_menu_item' == $post_type )
            return $slug;

        /*
         * Page slugs must be unique within their own trees. Pages are in a separate
         * namespace than posts so page slugs are allowed to overlap post slugs.
         */
        $check_sql = "SELECT post_name FROM $wpdb->posts WHERE post_name = %s AND post_type IN ( %s, 'attachment' ) AND ID != %d AND post_parent = %d LIMIT 1";
        $post_name_check = $wpdb->get_var( $wpdb->prepare( $check_sql, $slug, $post_type, $post_ID, $post_parent ) );

        /**
         * Filter whether the post slug would make a bad hierarchical post slug.
         *
         * @since 3.1.0
         *
         * @param bool   $bad_slug    Whether the post slug would be bad in a hierarchical post context.
         * @param string $slug        The post slug.
         * @param string $post_type   Post type.
         * @param int    $post_parent Post parent ID.
         */
        if ( $post_name_check || in_array( $slug, $feeds ) || apply_filters( 'wp_unique_post_slug_is_bad_hierarchical_slug', false, $slug, $post_type, $post_parent ) ) {
            $suffix = 2;
            do {
                $alt_post_name = _truncate_post_slug( $slug, 200 - ( strlen( $suffix ) + 1 ) ) . "-$suffix";
                $post_name_check = $wpdb->get_var( $wpdb->prepare( $check_sql, $alt_post_name, $post_type, $post_ID, $post_parent ) );
                $suffix++;
            } while ( $post_name_check );
            $slug = $alt_post_name;
        }
    }
    return $slug;
}, 10, 6 );