Blowfish is a free arcade game with cartoony graphics and sound, as well as a simple, silly, and surprisingly engaging schtick. You tap the screen to place and inflate blowfishes (which you can move while you're inflating), and your goal is to fill two-thirds of the fish-tank screen with gently floating--and often cutely expressive--blowfish before the timer runs out.
You have a limited number of fish per level, and each fish is vulnerable to fast-moving, spiky sea urchins while you're inflating it. The inflated fish bounce off each other and the urchins, but you can tilt your iPhone or iPod Touch to slowly maneuver the fish--and, ideally, box in the pesky urchins so you can inflate more fish in relative peace. At the end of each increasingly difficult level (you face more urchins with less fish), you can get a bonus for overshooting your two-thirds goal and for any remaining time, lives, and fish. The only downside to this game, aside from relatively unobtrusive ads, is somewhat limited replay value--but if you're looking for a free, fun, child-friendly game, Blowfish is definitely worth a try.