To clarify, I am a security officer in a school system and DW is a second grade teacher. We are no strangers to raising children and understand the in's and out's of raising children.
We have been to Disney with parents. I was not saying that all parents scream at their kids, what I was getting at is that kids are exhausted and over tired from being at the park and have at one time or the other taken there children out when tired or grumpy. Not all have tantrums. I was making the point that, being Disney people, we deal with a lot worse than the OP's situation when at Disney.
I also speak with the first hand knowledge of rude parents who take no responsibility for the action of their child or make the child take responsibility for themselves (older kids that is, not 2 year old's)
My belief is that many of the adults in this world have somehow gotten it in their heads that the "Me" has become the most important thing on Earth. And the "Others" can exist only if they don't infringe on them. We have forgotten that although people can be rude that is is even worse to be doubly rude and point it out.
In a nutshell, The mother should have made an attempt to hush the child out of respect of others in the area. The lady should have kept her comments to herself. Lastly, The Op should have ignored the old woman and went on with her shopping. We need to try to be the better person in situations like these.
I also understand that it is a hard thing to do when confronted by a tiny brain.

Can't we all just get along.