I have RealmResults that I receive from Realm like
RealmResults stepEntryResults = realm.where(StepEntry.class).findAll();
The answer by @EpicPandaForce works well. I tried this way to optimize my app performance and I find the following is a bit faster. Another option for people who prefer speed:
RealmResults childList = realm.where(Tag.class).equalTo("parentID", id).findAll();
Tag[] childs = new Tag[childList.size()];
childList.toArray(childs);
return Arrays.asList(childs);