She has always valued her personal space and used to burst into tears if another child ever got too close.
Now she is older and wiser 
she usually screams at them in the hope that she'll scare them off. This usually does the trick but recently she has also started hitting or pushing, especially if the child doesn't respond to the scream.
Obviously when this happens I remove her from the situation, talk to her about it and make her apologise to whoever she has hit. If she continues, she has time-out, which she hates. We are also giving her some other things to say when another child approaches (hello, what's your name? Would you like to play with me?) which she is sometimes starting to do but she usually still screams and hits 
Is there anything else I can do to discourage this? It's very difficult to monitor every single interaction when we're out as I also have a 6mo who I am invariably feeding when these incidents occur.
I was wondering about some sort of reward chart to encourage friendly behaviour but I'm not sure if she's too young. Has anyone else had any success with this?
It's just so frustrating as she keeps ruining what could be lovely experiences by being miserable and mean to the kids who could be her friends