I don't necessarily think she was treating your son like a dog by stepping over him. I would think most people would step over someone who was in their path, since the alternative is to not go to wherever it is they want to go, or step ON them, which obviously isn't going to happen.
She may have been in a foul mood that evening. If you tried to initiate conversation and she was nasty, then I think fair enough, decide she's a rude cow. If you tried to initiate conversation and she was distant and not terribly interested... that doesn't mean she's horrible, that just means she's perhaps not in the best of moods for whatever reason.
I'm assuming you just don't really like her... and this is just something else to add to the pile of "why I don't like her." I get that. I have people that I don't like, where any minor thing they do makes me furious. I just can say, as an outsider, based on your OP and subsequent updates in this thread, it doesn't sound like she's done anything so horrible she needs to permanently cut from your lives.