Starting with a falling leaf is a tribute to other films (Forrest Gump certainly, and I think A Bug's Life as well), plus establishes a bench mark, giving the flavor of the animation to come, in this case very competent.
The little girl on the tricycle establishes the humor level (quite high), and the preoccupied parents, plus baby sitter with boy friend, establishes our young hero's level of alienation from, and oppression by, his elders (also quite significant).
The plot is about an intergenerational bridge developing between someone just starting to connect with the opposite sex, and a more "been there done that" late in life guy haunted by past decisions. The older man is not entirely dead to this life, is just living a nightmare, brought about by his own sense of dedication to a lost cause.
The oldster, we discover, was trying to save a damsel in distress, but she was just too scarred by the teasing and cruelty of her own circus scenario, and she lashes out to her own detriment, resulting in her downfall and trapped "house wife" existence.
The old man is weighed down by the burden of his failed rescue attempt, and eyes all children with hatred and suspicion as the cause of his suffering, finding out only towards the end that they're actually a liberating influence.
Chowder is the brilliant Stand By Me dufus and sidekick who likewise overcomes obstacles, displays courage and loyalty, plus improved motor coordination, and therefore likewise matures through the arc of this film (albiet along a more comical trajectory).
And yet our ghost busting threesome are still children in the end, as are on some level the cops who came after them (but who were insufficiently believing in the reality of the dark side).
Having a girl on the team, likewise brilliantly played as stereotypically more adult-like than her peers, proves highly motivational to both boys. That crack about the uvula was hysterical.