Rough day at playground...
So, DD and I were having fun in the sandbox this morning, when she noticed a little girl who was a bit older (3) playing nearby. DD got up and sat next to the girl and continued playing w/ her own sand toys, but was looking at the other girl, I think, wanting to play together. This girl was the biggest brat ever - she kept throwing sand at my DD, and kicking sand at her as well; she pushed her twice; and generally was unfriendly. The mom's response was to tell her "no" but did nothing to remove her/gave no consequences when the bad behavior continued. My solution, unfortunately, was to pick DD up after the second push and leave. I whispered to her on the way to the car that that little girl was behaving badly, and tried to talk about how we had played w/ my friend's DD Olivia earlier that day, and emphasized what good behavior Olivia displayed. Poor DD kept talking about how "Big girl pushed me." 
Isn't it difficult when our LOs are learning life lessons at such a tender age. 1)Not everyone wants to be your friend; 2)People can generally display terrible behavior
Re: Rough day at playground...
Ugh -- I feel your pain!
My DD was a very quiet, non-aggressive kid. It was hard to deal with situations like this because there was no give and take -- it was ALWAYS my DD getting the short, rough end of the stick at the hands of other kids.
When my son came along, I thought it would be easier, but it was actually even worse, if you can imagine. He's a mellow, quiet kid like his sister, but being a boy, generally prefers to play with other boys. Parents of boys are even less likely than parents of girls to step in and put a stop to their own kid's bad behavior. Boys' parents seem more inclined to think that aggressive behavior in their sons is "cute" or "just how boys play."
Personally, I don't think it's ever okay or cute or normal to throw sand at another kid, push another kid down, or hit another kid, regardless of whether you're a girl or a boy! But I've run into plenty of parents of boys (some of them good friends of mine) who have waved off this behavior as "boys being boys!"