L.C.
It's such an awkward situation to be in, isn't it? Especially in public and around strangers. Some moms get pretty heated if you correct their kids. My feeling is different on that, within reason. If I take my kid out somewhere to play and I don't keep them within my direct line of vision and another parent sees them doing something inappropriate and corrects them, that's fine. If I have a problem with it, maybe I should have had my eye on them better. Having said that, most parents don't have that outlook.
Next time, I would just address the little girl they were mean to. Say, "It looked like those girls were being mean to you, and I am sorry they were like that. Let me introduce you to my daughter, so maybe next time you are both here you will have a playmate." Just validating her will make her feel tons better, and knowing there is someone there she knows will make her feel better about next time, too.