I found a work-around for this issue:
$parent = Doctrine_Query::create()->from('Parent p')->where('p.id = ?', 1)->limit(1)->fetchOne();
print_r($parent->getChild()->toArray());
$child = new Child();
$child->setParentId($parent->getId());
$child->save();
$parent->refresh(true); /* see note! */
print_r($parent->getChild()->toArray());
Note, from Doctrine's comments:
/**
* refresh
* refresh internal data from the database
*
* @param bool $deep If true, fetch also current relations. Caution: this deletes
* any aggregated values you may have queried beforee
*
* @throws Doctrine_Record_Exception When the refresh operation fails (when the database row
* this record represents does not exist anymore)
* @return boolean
*/