Copernicus
Industrial Grade Linguist
IMO, the belief that morality must be based on some kind of authority is quite natural for everyone, because we were all given moral guidance from authority figures during childhood. Gods assume parental authority as people become adults, so they tend to reflect the moral views of a self-organized society.
Surely a mortal would, in fact, seem like a helpless child from the perspective of a deity.
That depends on the nature of the deity that your imagination constructs. But the standard Christian god does not view humans as helpless, since they are supposedly gifted with the ability to defy his will and merit a trip to the fiery lake. Well, maybe that's not your favorite version of the Christian god, but we can sit around forever imagining gods, some of whom fit your criteria and probably many more who do not.