Re: doing random acts of kindness for ungrateful people, I feel it's still good to do minor things that lower the general stress level of the world. Suppose you didn't rescue the child or credit card of a jerk, and the mom has to chase the child in traffic or the jerk has to have their card replaced. They have a (possibly deserved) bad day, but then of course they may be the types to spread it around. The mom who had to chase her child is now late for an appointment, so she drives aggressively and cuts off two people, etc.
Anyway, good job! I would certainly have been grateful if somebody stopped my kid from running out the door. Likely the mom was too flustered to think of saying thank you.