Having grown up on the other side of the world, I don't really know much about bullying except for those snippets that I saw on television or read in books. I'm sure bullying existed in 3rd world countries as well, but for whatever reason - whether the country I grew up in was overly religious, too hungry, comprised of equally poor individuals, or something else - it just didn't seem to be as blatant as here in the US. I guess because of that, I've always seen this as a problem associated with privilege.
After that, I waited for the parents to approach me. They knew that I knew that they saw everything and they didn't stop it. Still, they chose to keep talking to themselves about the weather, and other trivial things. I could have left after that, but I wanted to give them the opportunity to talk to me. They could have apologized, they could have tried to explain, or they could have asked me for explanation for raising my voice at their kids. But no. The air was filled with apathy. I waited some more. Nothing. My husband said that I could have simply taken my daughter out of the situation as soon as I saw the sand kicking. In hindsight, I'm glad that I didn't, and that my fight-or-flee meter ticked the other way. Someone had to say something to these kids, and I had to hear the "Sorry" even from a 3 yr old. We only got out of the sandbox when the train nearby honked its horn and caught my daughter's fancy.
Now I can say that I've learned first hand about bullying, and how ugly it is, and how bullies can be raised by apathy. I'll try my best to make sure that my daughter doesn't turn into one. And if someone tries to bully her again, they can count on Mama bear here to come out growling.