Model_Child.php

<?php

defined('SYSPATH') or die('No direct script access.');

class Model_Child extends Model_Parent {
    
}

Model_Parent.php

<?php

defined('SYSPATH') or die('No direct script access.');

class Model_Parent extends ORM {

    public function __construct() {
		parent::__construct();
	}

	public function next_id($model = '') {
		if ($model == '') {
			$model = new ReflectionClass(get_class($this));
			$model = str_replace('Model_', '', $model->name);
		}
		$o = ORM::factory($model)->limit(0)->offset(0)->order_by($this->_primary_key, 'desc')->find();
		return $o->get($this->_primary_key) + 1;
	}
}

usage.php

<?= ORM::factory('child')->next_id() ?>