Dark? Separation anxiety? What age are we talking here?
"When I was a child, I spake as a child, I understood as a child, I thought as a child: but when I became a man, I put away childish things."
Look at all the "adult" men today, especially in America for some reason, whose desks are littered with action figures and whose offices contain a shrine to comic book superheros and Star Wars/Trek that would put the Sistine Chapel to shame. (Or women who seem to live in some alternate universe about their biological clocks, indulging infantile behaviors and attitudes, perhaps only to realize sometime in middle age the consequences, if not the cause, of their predicament.)
These are people who do not take their lives seriously. Someone clearly never told them that they're not 10 years old anymore.
One of the most valuable lessons you can teach a child is the ability to suffer. All who live suffer, and developing the correct attitude and orientation toward suffering is important lest you become soft and weak. Those who don't learn, those who flee from the suffering that the pursuit of the good often entails, suffer all the more and become depressed, miserable, and mediocre. A story in which the main character loses his toys can be a very mild, age-appropriate, narrative way of presenting the subject matter of sacrifice or loss. Growing up means parting ways with childish ways for the sake of something greater: adulthood. We do the same thing with pets. The death of a pet fish is a way of introducing a child to the reality of death in a way that's appropriate for the child's level of maturity. Maturation can be unpleasant, even painful, as the childhood attitudes and attachments and their familiarity are loosened. Film can also be cathartic in this respect.
(This is not an endorsement of Pixar or Toy Story, btw, only a general response to an apparent presupposition. Call it a rant, if you want. I vaguely remember watching the first Toy Story on TV, but I do not recall the ending and have only a vague recollection of the plot. For all I know, Toy Story may enable unhealthy dispositions.)