A kid was being physically aggressive at our apartment pool. I am not sure how to handle it?

I am 20. My husband is 22. We were swimming with my 15 year old brother in the apartment's pool. Our apartments are usually calm and quiet. Every kid we have met has been respectful except this one kid. He was about 6 or 7, and he was with his 4 year old sister. My husband, brother, and I were the only one's in the pool when this kid started calling us names. We didn't care about this. We just ignored him and carried about swimming. Then, he started throwing huge rocks at us in the pool that could kill someone if it hit one of us on the head. It almost got my husband. Well, my husband is disabled and has a couple of mental and many physical health problems because he was born prematurely. I am his live in care provider, so it really angered me that he almost hit my husband. We asked the kid to stop. I told him that he could really get in trouble with police if he hurt someone. He started mocking us and saying he wanted to knock us unconscious. His sister started yelling at us to stop being mean to him which I understand because that's how little sisters are. I explained to her that what he is doing could really hurt people. She said she would go get her parents because they were there with no adults. She came back and no parents were with her. We've never seen these kids before. We don't know where to talk to their parents at. The apartment offices are closed until Monday. We just decided to get out of the pool and go home. I'm not exactly sure what to do about this. Should I let it go, confront it with the parents if I can find them, report it to the apartments, etc.