Upsetting moment at nursery this evening. Thoughts welcome

Hi, I just went to collect my son from nursery and on the way in passed another parent who was telling off her little girl (about 3yo). The mum smacked her on her back and I (without really thinking) said たたくのは駄目でしょう and she said え and I said it again and walked off to get my son. Obviously abuse is wrong but also people have different norms for what's physically acceptable here, and I just feel terrible all round now. I feel sorry for the little girl and also a bit scared of seeing the mum again at nursery. I just wanted some thoughts on of you've ever been in a situation like this.

by lazylimpet