REST API & nested spaces

I’m currently Python scripting some Confluence to XWiki migration tools and I’d like to discover nested spaces in XWiki.

I know I can retrieve a list of all spaces by

curl -u "user:password'" "http://xwiki.example.com/xwiki/rest/wikis/xwiki/spaces/?media=json" | jq -r

But on a given space, e.g. Help, I can’t figure out how to get nested spaces below that space. This does not work:

curl -u "user:password'" "http://xwiki.example.com/xwiki/rest/wikis/xwiki/spaces/help/spaces?media=json" | jq -r

Is there a way using the REST API to get ‘subspaces’ without retrieving the full list of spaces?