I don't know, I guess I am in the minority here, because I figure if kids are less-than-perfectly-nice to my own daughter, it's a chance for her to learn a few things:
1. Not everyone will be nice to you, and that is THEIR problem, not yours, so don't take it personally.
2. If someone doesn't want to be your friend or play with you, go find somebody else.
3. If someone is being mean or teasing you, stand up for yourself and tell them to knock it off.
I was one of those kids who did get picked on a lot, so I'm not insensitive to it - if anything I am MORE sensitive to it, but I want my daughter to deal with it better than I did (which was to cry, tell the teacher, get upset, and then get teased some more). I am hoping eventually it will sink in and she'll learn some tools for not letting what others say or do impact her too much. I do tell her all the time how much I love her, how proud I am of her, and how wonderful she is, so hopefully she will believe me more than she believes another kid. I don't like to see it either but if DD doesn't seem that bothered by it, I figure she must be able to handle it okay. Like water off a duck's back.
And she is 4 years old, and in preschool, so I figure it's only the beginning. If they are only 2 or less, they don't even have real social skills yet so you can't take anything they do too much to heart.
ETA: There is a wonderful children's book by Max Lucado called "Because You Are Mine" and it does a terrific job of explaining that God loves us all because He made every one of us, and He does not make mistakes - so the only thing that matters is what He thinks, not what other people think. We read it a lot together.