something.php

<?php 
	public function get_children_ids($node_id, $tree_table, $left_col, $right_col, $depth = 1){
		$result = $this->EE->db->query(
		"SELECT node.*, (COUNT(parent.node_id) - (sub_tree.depth + 1)) AS depth
		FROM ".$tree_table." AS node,
		    ".$tree_table." AS parent,
		    ".$tree_table." AS sub_parent,
		    (
		        SELECT node.node_id, (COUNT(parent.node_id) - 1) AS depth
		        FROM ".$tree_table." AS node,
		        ".$tree_table." AS parent
		        WHERE node.".$left_col." BETWEEN parent.".$left_col." AND parent.".$right_col."
		        AND node.node_id = ".$node_id."
		        GROUP BY node.node_id
		        ORDER BY node.".$left_col."
		    )AS sub_tree
		WHERE node.lft BETWEEN parent.".$left_col." AND parent.".$right_col."
		    AND node.".$left_col." BETWEEN sub_parent.".$left_col." AND sub_parent.".$right_col."
		    AND sub_parent.node_id = sub_tree.node_id
		GROUP BY node.node_id
		HAVING depth = ".$depth."
		ORDER BY node.".$left_col.";");
		return $result->num_rows() ? $result->result_array() : array();
	}