am I wrong? long post

Off the T and the little boy who was hitting before him got upset cuz he still wanted to hit. My son was gathering the balls to put in a pile by the T when this little boy threw a fit put a ball on the T and hit it toward my son. I got pretty pissed off and shot the child's mother a look like get your child. She didn't do anything so I made my son stop what he was doing and come sit with me. When I did that she got after her son. Fast forward to the end of practice. I'm down on my knees changing my 20mth olds poopy diaper between some bleachers so no one can see and this Mom comes up to me and says " I'm sorry my son hit the ball at your son. He's only 5 and doesn't know better." Ummm excuse me your sons 5 and doesn't know better? Give me a break he threw a pissy fit and did it on purpose. I was so mad at that point and her apology lost all sincerity once she made an excuse for his actions. I just said ok, cuz what else was I supposed to say. I guess she didn't hear me say ok as I was down on the ground changing a poopy diaper so she stomped off mad. Am I wrong for being upset? Was my response wrong? I certainly wasn't going to say " oh it's ok" cuz it's not. I wanted to lay into this lady but but my tongue cuz I'm 31 and the drama is not worth the wrinkles.