kids semi bullied at park

Ok so I'm a single father of two boys, 4 and 5 years old. We decide to go to the park after several days of rain and being stuck inside. Once we get there there is only 1 other family it seems like, at least there's only 2 other kids, both boys, playing on the playground. My boys decide to go down the slide and start climbing up the playground. The last part is a ladder they have to climb. My 4 year old goes up and my 5 year old follows.Right as he gets his feet on the second one from the top, one of the other kids grabs his ankles and starts trying to pull him down. The parent of said kids is no where to be seen. But my boy actually yells at him to let go and he does. My boys get up and the one who was grabbed goes down first. Then the next thing I hear is my other kid who wasn't grabbed start screaming and crying. I yell up the slide for him to come down the slide. He does after I hear one of the other kids make a slapping sound, I'm assuming they hit my kid because he starts crying louder. He does get down the slide and hes just covered in tears. I tell them we are leaving and we get in the van and head home. the whole ordeal lasted 3-5 min. I just wanted to say this story and not hold it in. I didn't yell at the kids either I just took mine away from them as quick as I could.Thanks for reading. via /r/Parenting https://ift.tt/2OjoKSh