Well, that's what happens when the parents don't provide consequences for bad behavior. On the other hand, certain things are just kids being kids (i.e. overturning drinks). The trick is to differentiate between truly unacceptable behavior and simple kid stuff. Parents should also know what venues are and are not appropriate for their children.
That being said, I cannot stand people who like to cluck their tongues and roll their eyes when I am discplining my child who may be acting up. I've had plenty of dirty looks from people who were watching as I removed my child from a store or restaurant because they wouldn't behave appropriately. Actually, one lady tried to tell me I was being mean because I wasn't allowing him to throw a tantrum in public. How weird.
If you don't have kids, or if the children in question are not yours, please don't criticize what a parent does to correct the behavior. (Unless we're talking outright abuse or neglect, which is defintely an exception. )
I'd wait until you were done and then come up and thank you.
It could be worse... when one lady refused to discipline her daughter (a 6 year old) who was climbing up her, pulling her by the collar, and saying, "You WILL buy me that toy" my husband walked by saying, "You need to beat your kids more." Most women don't appreciate it when I tell them that story.
I usually give dirty looks to parents who don't discipline their kids ;)