Yes, she might have been having a bad day.
I had a very bad day one time when I was working in sales. I had recently had a miscarriage and a colleague had not heard and asked how the baby was doing.
A customer was a bit cheeky and I snapped at her. I did not swear but was rude and impolite.
I walked away, took a deep breath and went back to the customer and apologised for being rude. Thankfully the customer accepted my apology and that was the end of the matter.
There is no excuse for speaking a child like that. My own children sometimes drive me up the wall and I would NEVER use that kind of language with them.
Anyone who speaks to a customer or a child like that is not suited for work in sales.