Create a cheerful 9:16 vertical A4-style children's educational illustration poster about the underwater world for ages 2 to 5. Use a cute flat cartoon style, soft pastel aqua background, rounded friendly sea animals, clean thick outlines, playful bubbles, shells, seaweed, sand, a small diver child, and a treasure chest on the sea floor. Add a large colorful Chinese headline reading "海底世界 认知大发现" near the top, fully inside the poster with no cropped text. Arrange labeled callout boxes with arrows for sea turtle, shark, dolphin, clownfish, whale, seahorse, crab, seaweed, small fish, signpost, and beach ball. Each label should show Simplified Chinese, pinyin, and English in that order, using clean readable classroom-poster typography. Keep the layout spacious, bright, age-appropriate, and friendly, with accurate animal shapes and no brand logos, real copyrighted characters, or random extra text. Output a high-quality 9:16 image with crisp readable labels, clean composition, and GPT Image 2 ready detail.