Not my child; this is the question someone asked advice about at a mainstream board.
Stay with your child in such situations, and get between them and any other children if there's any sign of them hitting. Model the kind of behaviour with other children that you want your child to pick up on. Be really verbal "little girl down the slide and then it'll be your turn. There she goes 1, 2, 3 weee and now it's your go!". "That boy is playing with that ball. Shall we find a ball for you?"
Make sure you always have a soft toy or a book or something in your bag - acts as collatoral in potential toy conflicts. I know some groups of mamas whose children regularly go home with each other's soft toys, because that was the thing each child was happy holding as they parted. Charity shop toys are good for this sort of thing - I'd always be prepared to just give a 60p toy away to some other person's child rather than having a fight start.
Just in case you weren't anyway... you need to be right there with your child, helping them to learn about interacting with the world in a way that will make people love them. It's not a question of telling off, it's just that they've learned to walk and they've learned to talk a bit, and now they are in the process of learning to interact with strange children - they need your loving guidance in getting that right.