Although the answer comes 2 years too late, maybe it helps someone else still.
You can achieve this by a Java action. Something like this:
/**
* Provides the list of part category NPs subordinated to a category.
*
* @param category
* Category whose sub-categrories shall be determined.
*
* @return List of subordinated categories.
*/
private List<PartCategoryNP> getSubCategories(PartCategoryNP category)
{
final boolean reverseRetrieve = true;
List<IMendixObject> genericSubCategories = Core.retrieveByPath(getContext(),
category.getMendixObject(),
home.proxies.PartCategoryNP.MemberNames.PartCategoryNP_PartCategoryNP
.toString(), reverseRetrieve);
List<PartCategoryNP> subCategories =
java.util.Optional.ofNullable(genericSubCategories)
.orElse(java.util.Collections.emptyList()).stream().map(_subCategory ->
home.proxies.PartCategoryNP.initialize(getContext(), _subCategory))
.collect(java.util.stream.Collectors.toList());
return subCategories;
}