I am in the same boat with my just barely two year old daughter. So far I've chalked it up to the fact that she has to fight for my attention during the day. Every time a new kids starts the aggression seems to be more extreme but then subsides. I'm sure it has to do with the shared attention, the sharing of what used to be her toys, etc. It's tough and I feel bad for removing her from situations but she has to learn that it's not okay to be mean to other kids. I explain to her many times that if she wants mommy to stay home with her then this is how it has to be. I'm sure she doesn't quite get it yet but hopefully one day she will. Until then, I'm open to ideas/suggestions as well!!