It's a thin line - you want them to respect other children & their property, but it can be hard to watch your kid be pushed around by others too. I wouldn't have encouraged my DS to retaliate with aggression to other children, but as soon as he was old enough to vocalise his unhappiness with a situation, I taught him to say "Stop that" or "no" firmly, and to get an adult to intervene. If my DS were to do anything to one of his friends that made them uncomfortable I would tell him off, but if they are fooling around, rolling about on the grass, then I'm happy with that now.