Why did this woman seem so terrified?

One of my favorite Halloween memories is having a passel of the grandkids here. This was was about 8 or 9 years ago. There was a knockin’ on der door. I opened it with a smile, a bowl of candy in my arms. The little girl, 5 or 6, looked at me, then looked around me, and there was a little kid’s Halloween party going on! She just brushed right passed me and came in the house!
I was like, “Oh! OK! Comon in !”
Her mom was just mortified. I told her it really wasn’t a problem and invited her to come on it. She looked at me in sheer terror and said “NO!” and kept hissing for her daughter to come to her.
I finally went and took the girl by the hand and mom scurried away with her. What did she think I was gonna do with the child?

What would you have done if you were that mom?