What's It to Ya?!

Tact is not something I was naturally born with.  Working as a speech aide in a public school has taught me to think before I speak.  But sometimes I wish I could just let 'em have it.  "Your kid won't get better unless you work with them outside of school!"  But I'd prefer to keep my job.
A few months ago Mike and I were out on a date and we swung by the mall so Mike could check something out real quick.  I parked by the curb and saw a young woman in her early 20's dragging her child, a boy no more than 5 years old, by the arm.  He was screaming "Ow! Ow!" as most kids would in that situation.
When they arrived at their ride, parked along the opposite curb, the woman intentionally swung her arm holding the boy and the boy's head banged into the side of the car.   He then stood there crying, holding his head.  She proceeded to swear at him, picked him up, and plopped him into the car.  Having been in Rexburg, Idaho for such a long time (often called a "bubble" keeping us from harsh realities of the world) I was appalled at this mother.
Mike got back into the car and I told him what I saw.  The mother was still struggling to get her son buckled up and Mike said, "well do something about it."  The thought had crossed my mind but I get so nervous in confrontations, plus who was I to tell her what was right and wrong...right?  Well I don't know what got into me.  I rolled down the window and said, with what I thought was a sympathetic look on my face, "Try to be more gentle.  He hurt his head."  The look she shot me was the classic "who are you to tell me!" look.  I cowered back in my seat as she approached the car yelling at me, telling me that I have no idea what kind of day she's had, that her kids are brats, and so on (you can throw in your own profanities to make it more accurate).  I honestly thought she was going to reach through the window and slap me.
She was right though, I really had no idea what her life is like, what kind of crap she had to deal with today, or her children's degree of normal behavior.  I felt horrible for being "that" person to critique another woman's parenting.  I quickly responded by telling her that I was just worried because he hit his head on the side of the car (and it wasn't a tap) and she responded with, "So what if he did! He had it coming!"  Oh.  Well, then I felt like calling child protective services and turning that woman in!  I don't care who you are, what kind of day you've had, or how bratty your kids are, no one "deserves" abuse like that!  Don't get me wrong, I'm not anti-spanking, but I do believe that it shouldn't be done out of anger.
But really, where do you draw the line?  I understand having screaming kids in the grocery store because you don't give them what they want, or kids throwing tantrums while collapsing on the floor screaming bloody murder in the middle of the mall because they want to walk where they want and not have to hold your hand (yea, been there).  I understand it isn't necessarily the parent's fault.  But by golly I draw the line at injuring your kid in public and saying that they "had it coming".  Who's going to speak up for the kid?  What would you have done?  Do you think there's a better option?  Should I have just kept my mouth shut and minded my own business?
All in all, what's done is done but it did bring the question to mind, where do you draw the line?