Thats an assumption I wish to correct. Sadly, domestic and child abuse DOES happen within a church environment. Which is even worse because either people are aware of it and stick it under the carpet like a shameful secret (esp if the abuser is an elder or pastor) or they have no idea because the abuser is so good at lying and putting on a fake image of 'christian perfection.'
I was abused by my step father, and while I was the only one in my family to have a noticeable relationship with Jesus, my step father enjoyed prancing around, going to church, reading his bible in front of people in public and giving people the impression he was an awesome dad. He had no issues with taking me to church (as a way of watching me and having a level of control over me) then going home and beating me for not doing something right and telling me that singing loudly or raising my hands showed a lack of decorum. when I left home and made it public what he had done to me - no one believed me - especially not anyone from my church. when they finally came around they wanted to keep it hush hush because they were worried their diocese would find out they preferred the opinion of an abuser over a victim.
Abuse is a complicated and many faceted area is society that Satan relies on as it always runs in cycles. I commend you for what you did and for your bravery in sticking up for that child. I only hope that the child wasnt hurt even more for your actions in publically embarressing their parents. But parents need to know that society does not accept that kind of behaviour towards children. A word of warning and advice though. I would refrain from accusing parents that you only randomly see in supermarkets. You dont know their circumstances surrounding them or the child and good parents can slip up occaisionally because they were tired or stressed. This doesnt condone it but, often a parent knows when they make such mistakes and try their best to improve. On the other hand, if you know of a family and see them frequently enough to suspect abuse is occuring, then definitely do something. Often its a domestic situation as well which will be harder, if the woman is battered and under the control of her abuser and is having her children used against her as blackmail. If that is the case, you need to counsel her as a friend and convince her to have her children taken out of that environment into safety so that she can deal with it appropriately without worrying about harm comming to children.