Based on John Nickle's children's book, this computer-animated comedy starts slowly but builds into a rousing adventure capped with just the right measure of sweetness. The movie's early sequences are a bit talky for younger audiences, but once the tweener hero has been shrunk by a magic potion and kidnapped by the ant colony he's been terrorizing, writer-director John A. Davis (Jimmy Neutron: Boy Genius) shows heaps of invention. The amusingly skewed microcivilization presents tall challenges for the now miniature boy, who learns teamwork through scavenging jelly beans and bravery in the belly of a frog. Julia Roberts, Nicolas Cage, Paul Giamatti, Lily Tomlin, and Meryl Streep are among the voices. PG, 88 min.