Seriously? In this case it's more like there was a reward for their son, and this guy found their missing dog. Either way, you would complain about not getting a reward for finding a missing child? Full stop. Prior information doesn't matter, what was the alternative? If you had known there was no reward, would you have acted differently?

I would complain if a reward was offered and I didn't get one. I don't think that makes me a bad person. I'd save the child either way.