We try to keep an eye on our kids as well and find that there are times our kids get picked on as well. Just the other day we took the kids to McD's w/ a playround - a boy about 8 yrs old went up poking at my 4 yr old... to the 8 yr olds suprise, my son pushed him letting him know enough. When the boy went to react we told both of them enough. When the 8 yr old notice we saw the whole thing him & his older brother walked away. Thus, all the kids went there own way and started playing again. But I have a feeling it would have gone a bit farther if we wouldn't have seen it all take place.
We try to teach our kids to share & hope that they "stand-up" for eachother, but also want them to know how to walk away. Currently they are 2 1/2, 4 1/2 & 5 (6 in less then 2 weeks), so we are still trying to teach them, but all in all they do ok.
Personnaly, I feel a lot of parents don't keep an eye on their kids... we have 3 yr old kids playing in the streets here and parents inside the house doin who knows what. An elderly lady we know has had kids (ages 4 - 10) come to her house to ask if the can play football in her yard (she has a big yard) as late at 10:00 at night. She tells us all the time how refreshing it is to see how we are with our kids... although they are far from perfect & they do act up (they are humans & kids) all in all they are good kids. Most of the time they are respectful, they have a set schedule& rules and they go NO where without mom or dad watching over them well except the backyard - which has a 6 ft wooden fence around it, so we know they are safe there.
So you know, it does bother me to see 4 yr olds playing in the street & kids running around at 10 at night. It also bothers me to know parents are leaving their kids at home alone to walk the other kids to school. I don't let my house out of my sight if there is a kid sleeping in it. What it basicly boils down to is that everyone sees parenting in different ways - as to what is acceptable & what isn't. So, at this point, you know what is acceptable in your eyes, you have 2 chooses - confront the other parents about your feelings or don't allow you kids to play with the parents' children you have issues with.
Good luck and in my eyes most parents the are protective out of love aren't overprotective and don't overreact.