Of the Miyazaki films I've seen, Ponyo (2008) is the first I can imagine an American child enjoying, even without an adult telling them why they should like it.
It feels lighter, more weightless, more improvisational and less death-0bsessed than film such as Spirited Away, Princess Mononoke or Howl's Moving Castle.
Perhaps that is because the characters are floating much of the time, either because they began life underwater, or because the water is overtaking the land for much of the story. But there are none of the ominous older magic-oriented characters often found in Miyazaki who bring cruelty and destruction. Even the wizard in this story is essentially benign and brings resolution and reconciliation to the story.
And the strange inexplicable elements in the story--the fact that Ponyo never really does look like any kind of fish, Ponyo's marvelous pursuit of her little friend on land by running on giant fish that look like waves (illustrated) or even the whole world's lack of concern or upset that the whole landscape has been flooded--all have a plausible dream logic to them. These things would literally happen in dreams; not the artificial "dream" designation that Disney uses for materialistic fantasies, but real bits of confusion that your brain brings together in a weird narrative that makes perfect sense until you wake up.
Moreover, the film is as brightly colored as a preschool classroom; in fact, it has much more of a preschool feel than his other works, and thus perhaps the optimism, gaiety and the simple faith in friendship that small children bring with them. And for us grownups, perhaps the most delightful part of this fairy tale is that it doesn't make sense, it's not consistent, and it hasn't been organized so as to teach a moral or a lesson--it's a joyful hodgepodge to be enjoyed as an experience. Because Miyazaki is an artist, not a schoolteacher. And school is out.