I wouldn't assume the mother actually believes in reincarnation or wants her son to come back as an animal or that she's scarred him for life by saying something that implies that he will die some day.
There is maybe a "passive-aggressive" kind of edge to her comment, but tired parents do what they can to make their children a little more thoughtful.
The whole tribal "them against us" thing gives people a chance to unload against the presumed hippy, New Age mother, and that explains what's going on.