Did you give the mother a chance to correct her child?
Mom should have been the person to correct her child, if nothing was forth coming then a simple "uh-oh, it's not good to climb under the wall, besides it so dirty down there".
This had nothing to do with a 2 year old's lack of respect for your space. She acted on impulse, something all 2 year olds are still trying to master. She's not going to remember climbing under there or ever seeing you. Lets say for the sake of argument that she does.....you will probably never see her again and if you do what is she going to say? I mean the logic makes no sense to me.
Dang if i had a dollar for every little one, mine included, that has climbed under a stall. I would be one very rich woman.
ETA..........................................
I find it funny how so many moms are assuming that the mom wasn't watching her kid. In a blink of an eye a kid can be anywhere and do anything. Nowhere did she say mom wasn't watching her kid. she said that mom got upset with her choice of words....I would have too.
Yes, a 2 year old is capable of learning some manners, but the key word is learning. They are still learning and with any learning process it takes time, patience and understanding.