What do you do when another kid pushes or grabs items from your child's hands? How do you teach your kids to handle it?

I know this happens all the time, but now that my daughter is 2, she is more aware and going to start pre-school soon. This has happened a few times in her classes and activities, but I have always been there to help her through a situation. Now that she is starting school and drop off classes, I'm wondering how others tell their kids to handle this. Up until now, she has been taught to share and play nice, gently, etc...She know she won't get away with bad behavior in front of me. It seems wrong and confusing to now tell her to say, "that's mine!" or to push back when a kid pushes her.

blargh...parenting is so hard. This morning an older boy in her preschool prep class pushed my tiny 2 yr old --twice! My sitter told him to stop and his mom said, "he's just a kid." If Zoey did that, she would get a "talking to" from me. Is it me, or is that just not acceptable behavior?! It breaks my heart to think of that happening when an adult is not with her.

Parents, how d you handle these situations?