Omg how horrible for your ds and you I would have said something similar to you. Like 'do not touch my child how dare you!' and if she were to say anything about my child hurting hers I would have said 'yes but we're the adults and supposed to be the role models' and walked off. Honestly she does sound like an absolute feral b***h. I can't believe she hit your child and said what she did. Sounds like she is taking her own insecurities out on you.
One time when I was shopping dd was acting up abit she was a lot younger about 2 and an older lady was talking to me quite pleasantly but then she went to grab my dd and smack her hand. I quickly said oh no don't touch her she said sorry and I walked off. It's so hard to believe when it happens.
Eta - I agree go ask macdonalds for survaliance and charge her with assault.