I don't think a child with a disability or something like that is karma. Nor do I feel there is a reincarnation karmic bank account. How the heck is an infant, just developing conscience to know what the action and what the consequence? How the heck are people in bad circumstances to determine their actions of an unknown past life?
I do believe in cause and effect though. Where I am today is the consequence of actions committed by me and others in my past. Similarly tomorrow will be the consequence of another action. These consequences also give us a conscious understanding of what sort of actions benefit us and what sort of actions detriment us in the course of life.
Human society, the natural order in nature all function on cause effect. There is an action and it follows with a consequence. In the grander scheme of things I believe the entire universe is governed by laws of cause and effect. The only thing is we don't have a full understanding of how it plays out. We don't even know how the other person will react if we do something, how can we possibly begin to understand how every action will play out in the cosmos.
The way I see it – everything is karma. But its not like oh my God your karma is good that is why good things are happening or Karma is bad so bad things are happening. It is more like , if you try to understand the scheme of things you will understand how and why you are here and figure out how to move forward. Doing wrong to someone might end up having something bad happen to you – because either they, their supporters might try to get back, your supporters my want to teach you a lessen or your own subconscious guilt will play a role in affecting you.