Other children seem to be rude to my daughter-- why?

I so feel you.. I have a three year old and while she's not experiencing such negative reactions, the older kids just often don't want to play with toddlers and it's heart breaking to witness. There are a group of girls similar to my daughter's age range and they are EXACTLY like those mean girls you describe (and these girls are only 3/4 yo.) There mothers are in the playground, too busy drinking coffee and gossiping to intervene and correct their children's terrible manners. I figure, I finally figured out where those really mean and cliquey girls in high school came from. I leave the park or try to "be" the party (I find when I'm really exciting and doing something fun and energetic with my daughter, the other kids want to join us and do what we're doing.. changes the whole dynamic.. they like the adult attention and they're giving my daughter positive attention.) We can always go to another playground and find nicer kids.. they're out there... with parents who will parent if their children are ill mannered to another child.