The specific call you asked for indeed requires artifactory pro. What this means:
before working on it you should compare the cost of implementing it vs. the cost of buying it (make sure you consider possible chance of wasting time on implementing other "pro" API features)
If your time still results to be cheaper and (if I understand the intent correctly) given artifact name and group id, you want to get list of versions.
what you can do is write a method that implements that "pro" api call. meaning:
children_list
=/api/storage/<repo>/<group id split by slashes>/<your artifact>/ | grep folder ="true"
- you can (should?) check whether the children actually have subchildren (built artifacts)
- go over the list items, and create the list of versions with subchildren
hope I'm not too confusing :-)