<?php
class MapsPage extends Page {
   static $has_many = array (
      'SmallMaps' => 'SmallMap'
   );
   public function getCMSFields() {
      $fields = parent::getCMSFields();

      $gridField = new GridField("SmallMaps", "Maps list:", $this->SmallMaps(), GridFieldConfig_RelationEditor::create());
      $fields->addFieldToTab("Root.Maps", $gridField);
      
      return $fields;
   }
}

<?php
class SmallMap extends DataObject {
   public static $has_one = array(
      'MapThumbnail' => 'Image',
      'MapsPages' => 'MapsPage'
   );