you did nothing wrong but your use of the word "confronted" makes me wonder what approach you took with the parents...if you came to me ready for a fight to tell me what my child had done I would probably have an issue with it, if however you came over and said something along the lines of "just so you know..." I would thank you for your information, be aware that I needed to talk with my child about proper behavior and keep a better eye on them while out...I think parents that don't want others to correct their kids should keep a better eye on them so another parent doesn't need to.
As for what to do now, don't let your daughter play in a group with that girl, if the girl seperates herself from the group when your daughter comes out to play try to be fair about how much time the other little girl has with the other kids in the neighborhood...I think 6 and 3 is a bit too much of an age difference and they shouldn't be playing together to begin with but I understand when you have a big open area and a wide range of ages it happens and can work out quite well...it does sound like the kids are starting to seperate themselves on their own, let it happen, kids will get into their own circles and all will be fine with them.
As for dealing with the parents, leave it be, don't be ashamed, don't avoid areas where they are as that will give the impression that you feel you were in the wrong. The other parents in the neighborhood don't really care what is going on and are right to stay out of the "quarrel." Speak with parents but don't bring up the incident unless they do, maybe they want to hear the other side. Not everyone in a neighborhood is going to get along, sometimes the kids get along but the parents don't sometimes it is the other way around...it will "blow over" at some point, something else will happen and that will be her topic of discussion until the next dramatic episode...