The CAML for the query easily enough includes a ParentID reference. However, if you’re spelunking around in SP 2013 using the OData services, you might have a hard time finding the ParentID field.
However, if you just issue the query:
//web/_api/Web/Lists/getByTitle('TaskListName')/Items/?$filter=ParentID">https://<server>/<mp>/web/_api/Web/Lists/getByTitle('TaskListName')/Items/?$filter=ParentID eq ‘101’
You’ll be able to retrieve all Tasks that have task #101 as their parent.