[SalesForce] Can’t access child Data Categories

I'm trying to get all Data Categories from an Article Type through the varius describe calls I've found in the documentation such as https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_class_Schema_DescribeDataCategoryGroupStructureResult.htm

This is the code I'm using, running it through Anonymous Apex

List <DataCategoryGroupSobjectTypePair> pairs = 
      new List<DataCategoryGroupSobjectTypePair>();

DataCategoryGroupSobjectTypePair pair1 = 
      new DataCategoryGroupSobjectTypePair();
pair1.setSobject('KnowledgeArticleVersion');
pair1.setDataCategoryGroupName('MyArticles');

pairs.add(pair1);

List<Schema.DescribeDataCategoryGroupStructureResult>results = 
      Schema.describeDataCategoryGroupStructures(pairs, true);
          for(Schema.DescribeDataCategoryGroupStructureResult r : results){
              System.debug(r.getName()); //'MyArticles'
              Schema.DataCategory[] topCategories = r.getTopCategories();
              for(Schema.DataCategory c : topCategories){
                  System.debug(c.getName()); //'All'
                  Schema.DataCategory[] subCats = c.getChildCategories();
                  System.debug(subCats); //empty, why?
                  for(Schema.DataCategory sc: subCats){
                      System.debug(sc.getName()); //Never reached
                  }
              }
          }

My Data Categories are structured with one 'All' Category at the top level, which I can find using getTopCategories(), but when I call getChildCategories()on the result, I get an empty list, despite the fact that I have 5 child categories set up under it. I made sure Data Category Visibility is on, but I still get no results. Is there any other reason I wouldn't be able to see these subcategories?