I went to my cousins house the other day, we grew up close but not as close anymore, own families etc. We have a catch up and a chat once every 3/4 months.
She has a DD who is 3 years old and a new small puppy.
Her DD was playing extremely rough games with the puppy and I was getting more and more anxious and also annoyed by this, her mother, my cousin didn't seem to bat an eyelid.
Then 10 minutes later her DD picks the puppy up with both hands around the puppies neck 
I jumped up from my chair and said, firmly but not shouting, "no no... put him down quickly" as I went to her and took the puppy from her. Her DD didn't seem to care, just looked a bit sulky.
As I returned to my chair my cousin also looked sulky and after another 15 minutes I left as I felt awkward and as though I was no longer welcome.
I then received this text later in the evening which I have not replied to yet. (Changed names to DD and puppy)
I didn't have the words to express it earlier but I was really upset with how you treated DD. She doesn't know you well enough to be told off by you. She is 3 years old and we have only had puppy for 2 weeks so DD is still learning how to play with him. She doesn't dare play with him at all now incase she is told off again. You had no right to do that.