Main mistakes:

I tried to extend modObjectGetListProcessor  and not modObjectProcessor, for some reason I could not set the root node to results as other elements allow.

Known this and looking at core/model/modx/processors/security/group/getnodes.class.php make things clear.

So that's what I came up with.

/**
 * Get a list of Tags
 *
 * @package modtours
 * @subpackage processors
 */
class TagGetNodesProcessor extends modObjectProcessor {
	public $classKey = 'Tag';
	public $languageTopics = array('modtours:default');
	public $defaultSortField = 'name';
	public $defaultSortDirection = 'ASC';
	public $objectType = 'modtours.tag';

	/*
	* Get list by parent
	*/    
	public function process() {

		// Get parent ID
		$parentId = str_replace('n_ug_','', $this->getProperty('id') );

		// Get child tags
		$tagList = $this->getTags($parentId);

		// Write list
		$newList = array();
			foreach( $tagList as $item ){

			// Check if children exist
			$itemChilds =  $this->getTags($item->get('id'));
			if( !empty($itemChilds) ){
				$leaf = false;
				$type = 'dir';
			}else{
				$leaf = true;
				$type = 'file';        	
			}

			// Write new node item
			$newList[] = array(
				'text' => $item->get('name'),       
				'name' => $item->get('name'),       		    	
				'description' => $item->get('description'),       
				'parentid' => $item->get('id'),
				'id' => 'n_ug_0' . $item->get('id'),
				'leaf' => $leaf,
				'type' => $type,
				'cls' => ''
			);
		}    
		return $this->toJSON($newList);
	}
	/*
	* Get results of tags by parent id
	*/
	protected function getTags($parentId) {
		$tagList = $this->modx->getCollection('Tag',array(
			'parentid' => $parentId
		));
		return $tagList;
	}	
}
return 'TagGetNodesProcessor';

And then just create your new tree Object

modTours.tree.Tags = function(config) {
    config = config || {};
    Ext.applyIf(config,{
        title: _('modtours.tags')
        ,id: 'modtours-tree-tags'
        ,url: modTours.config.connectorUrl 
        ,action: 'tag/getnodes'      
        ,root_id: '0'
        ,root_name: _('Tags')
        ,rootVisible: true
        ,ddAppendOnly: true
        ,useDefaultToolbar: true        
    });
    modTours.tree.Tags.superclass.constructor.call(this,config);
};
Ext.extend(modTours.tree.Tags,MODx.tree.Tree);
Ext.reg('modtours-tree-tags',modTours.tree.Tags);

Which you can use like

,items: [{
    xtype: 'modtours-tree-tags'
    ,preventRender: true
}]

Ope this helps..