Get all TV's where parent (category) is X

$modx->resource->getTemplateVarCollection(array('id' => 6));

$modx->resource->getCollection('TemplateVars', array('id'=>'6'));

$criteria = $modx->newQuery('modTemplateVar');
$criteria->where(array(
    'category' => $categoryId,
));
$tvs = $modx->getCollection('modTemplateVar',$criteria);
foreach ($tvs as $tv) {
    //do seomething with each TV
}