WWYD - “mean” kids & no parents around

I have a 4 year old son. He is very friendly. Willing to play with anyone, shares well and open minded. We had an incident today at a playground that is down the street from our house. We go there often. We had been there for about 15 minutes and a van drove up, 3 kids got out, ages range from 3 to 8 maybe. The adult(assuming the mom) stayed in the vehicle. About 10 minutes later, she got out with another kid, maybe almost 2. The mother stayed for a minute then left all of the kids there and walked across the street to a church. After she left, the kids moved from the swings to the slides. My son was also at the slides. All seemed to be ok and playing well together until one of the boys maybe age 5 blocked the slide and another boy(assumed to be brothers, all seemed to be siblings) blocked the ladder to get to the slide. That area is about 12 feet off the ground. At this point another van had come, dropped 2 kids off and the mother went to the church and another vehicle was just arriving and dropped off another kid and the adult stayed in the vehicle. So there are 7 kids that arrived in vehicles and 2 girls ages maybe around 10 walked up. So there are 10 kids there and no adults for any of the other kids. I was the only adult there. I am standing about 10 feet away from the playground equipment. My son brought a stuffed animal dog with him. One of the kids threw the dog on the ground and the other 2 boys wouldn’t move from the top of the slide or the ladder that leads up and down. My son started crying and screaming that he is trapped. They wouldn’t let him down or anyone else for that matter, but they were all related or knew eachother so I wasn’t too concerned with any of the other kids as they all had adults choose to just dump them there. I told my son he needed to come down, whichever way he could. I told the boys to move out of the way. One of the oldest boys whispered something to my son twice. For 3 minutes this continued. Finally one of the other kids pushed his way through and my son followed and we left.

I was pretty angry by that point but kept in to myself. I told my son that no one is allowed to trap him like that and he should have pushed the boy at the top of the slide out of his way so he could get through. My son’s response was that the other boy might get hurt. Oh my goodness, I am a mama bear and my natural instinct was to climb up to the top of the slide and fling the kid out of the way. I would never do any of that but I am just so angry there were NO adults there.

So what would you have done? Please provide me with your perspective.

Add: thank you for putting it into perspective. I should have spoken with the absent parents. We had never seen these kids before and they all clearly knew eachother. We live in a small town and although I am hyper aware there could be people around that want to hurt children, some people take small town living for granted and in my opinion become too careless. Anyway, I acknowledge that many people do feel that way here in this town and figured the dumping of these kids at the playground must happen often. So I assumed it wouldn’t even faze the mothers if I did approach them about it. In the future, we just won’t play with these kids but if a similar situation does arise, I will either speak with the parents or call the police so they can speak to them. Thanks again.