You need to get the original document with getObject() not getOne(). The getOne() method is for getting a related object of an object you already have and only in cases where there can be only one such related object -- e.g., $resource->getOne(’Parent’).
The $modx object doesn’t have any related objects, so you never call $modx->getOne() or $modx->getMany() -- those methods are only used with objects that represent tables in the DB (modResource, modUser, modTemplate, etc.).
I think your code will work with this instead of the first line.
$parent = $modx->getObject('modResource',$id); // parent with many Children already