There are many toys that expand in water available for kids of all ages. They tend to be called many different things, and can usually be found in the science section of a toy store. These toys are made of a combination of hydrogel and another form of polymer. Most will shrink back to its original size when it is removed from the water.
The Magic Expanding Grow Growing Water Dinosaurs start out at around 2 inches and expand up to 600 percent when they're left in room temperature water for up to 72 hours. The Growing Sealife toys by Steve Spangler Science has similar expanding animals that include grow frogs, sharks, dolphins, and many other forms of aquatic life.
The "Grow a Boyfriend Mr. Right" toy is a novelty item. It grows up to 600 percent of its original size and, according to its marketing line, will "never complain or argue." A "Grow a Girlfriend" version is also available.
Some grow toys serve a more functional purpose after they have been expanded. Your own logo can be added to the official Magic Towel; it will materialize after the towel is soaked underwater. Magic T-shirts work in a similar fashion - soak it in water, and it will expand into a full-sized T-shirt that can be worn.
Nature Pavilion, which specializes in educational science gifts, had nearly a dozen toys that expanded in water in 2010. They included "eggs" for alligators and chickens, a penguin in a keychain, a water hermit crab, a magic "volcano" and a mini aquarium.