You Reap What You Sow

I've been doing a lot of thinking about the concept of karma.
I was watching a rerun of Wifeswap this afternoon (don't judge me), and the episode featured a Rastafarian dad who had very little money in his bank account, but kept saying he was putting his savings away in a "karma bank," and that it would be Karma/God (or "Jah," as he referred to a higher power) who would provide, instead of an actual monetary savings account. 
While this man was, throughout most of the episode, one wild Froot Loop, the notion of building up good deeds, kindness, charity, selflessness, and so forth, in a sort of "karma bank" intrigued me, and isn't too far removed from what most people believe in, whether it be in religious form or simple human decency.
I'm a big believer in the idea of karma.  For me, it's a very simple concept--you treat others as you want to be treated, and in general, they will reciprocate.  It's not entirely perfect because others have free will/agency, but as a rule, it's very true. 
Today, the hubby and I had the opportunity to take the Goob to McDonald's for a one on one dinner (yeah, I know, we've been eating out...I so failed at my challenge), and we observed two very noisy teenaged girls, who seemed hellbent on making fun of everyone and everything.  Whether it be the workers, or people who went in and out, nothing was out of bounds.  A very mentally disabled young man came in to enjoy some milkshakes with his grandmother, and I assumed that these girls would have the common sense and decency to spare him their taunts, but he received the worst of them.  They mocked his walk, his voice, his grandmother, his shyness--you name it.  I watched in horror as his face burned in shame, as his grandmother blushed in embarrassment and sadness for her loved one.  She tried to gently scold the girls when they left, and they simply laughed in her face.
I looked at Goob, and I thought to myself that, foibles and all as a parent, that I instill this basic notion of simple kindness and common sense into him.  I was also embarrassed that I didn't speak up, instead choosing to watch in pity and disgust. 
I know it sounds awful, but as they say, karma is a real you-know-what, and I hope that sooner rather than later, these girls will be clobbered by the karma train.  I firmly believe that we reap what we sow, and, as my aunt said to me, karma has a way of providing a "punishment" or "reality check" far better than we ever can.  How sad that we sit there instead of doing something about these situations.  How I wish I had told those girls a thing or two.  But, I sat there.  Did sitting there add anything worthwhile to my "karma bank?"