I hate criticizing other parents. Mostly because I have a healthy fear of the law of karmic retribution, since every "Well MY Kids Will Never Do THAT" type of shitty thing I said out loud (and a few I thought) about other parents in more advanced stages of parenting than me, I have had come up and bite me so hard in the ass I'm suprised I have cheeks to sit on. I'm sure I have my comeuppance yet again now that my first three kids are all teen/tweens, since I was the very best parent ever to teenagers when I only had sweet little kids. 
Sometimes kids are little shits. Any parent who claims that their child has never been one is (IMO) lying or delusional. This kid tonight was not being a brat, he was just being a 2 year old who was in an inappropriate environment. That's really what turns it into a peeve for me. Toddlers drop to the ground. All of mine have done that in embarrassing situations (parking lots, the aisle of the airplane, Target, naked in the middle of the locker room floor while soaking wet after swim class, ect). Hell, my twins were bolters so I even tried those leashes (but then they promptly ran crosswise from each other and clotheslined each other full speed the first time I tried those fuckers out).
I understand the difficulties and lack of control over children's behavior. It still doesn't mean I don't get annoyed when a parent chooses to bring their child to an area where it is clearly marked that they're not supposed to be, and then willfully chooses to ignore then while they are doing things that are dangerous/part of the reason why children that age are not allowed to be in that area.
That really has nothing to do with luck.
I have kids that are not picky eaters and thus will eat almost anything I set in front of them without complaint. That's luck. It has nothing to do at all with fantastic parenting on my part, because I did nothing at all parenting wise to receive that. I have a good friend whose child has had suicidal ideations and attempts since he was 8 (he's now in his mid-teens). Also luck, because she is a much better parent than I will ever hope to be, and this terrible thing is happening to her and her child that is neither of their faults.
Bringing your child to an inappropriate and marked place and then ignoring them to the detriment of the many other people using that space appropriately is not luck though, that's a choice. Deciding to do that and then to get pissy when someone speaks to the kiddo to help (I didn't know who the parent was until I did that) is also a choice.