Ponyo or My Neighbor Totoro would probably be best for a first choice. Kiki's Delivery Service and The Secret World of Arriety would be better for a (slightly)

Ponyo or My Neighbor Totoro would probably be best for a first choice. Kiki's Delivery Service and The Secret World of Arriety would be better for a (slightly) older child. Spirited Away or Howl's Moving Castle for slightly older yet.