$array = array(
            0 => array (
               "id" => 1,
               "parent_id" => 0
            ),
            1 => array (
               "id" => 2,
               "parent_id" => 1
            ),
            2 => array (
               "id" => 3,
               "parent_id" => 1,
            )
            3 => array (
               "id" => 4,
               "parent_id" => 0
            )
            4 => array (
               "id" => 5,
               "parent_id" => 3
            )
         );

find_children($array, 1);
// output : 2

find_children($array, 3);
// output : 1