It doesn't involve killing kid to finish the quest. But if you wish you can kill the kid. Also, you don't even need to kill his father to complete the quest. You can finish it by lying that you kill him...
End story is you discovered all train stations if you ever need them, because buggy is 15k caps and faster than train and you dont need to wait for it or buy ticket...
On other hand even if you mark the rails on map, player still needs to explore the map and discover the train stations because of fog of war...