A sponge with a scratchy side, soft scrub with bleach, and a hose. It works wonders. I'm a big yard saler and this is how I clean all plastic toys. It take a little elbow grease, but it will look new when you're done.
A scrubbie sponge and some antibacterial dish soap or all-purpose cleaner. If it was really nasty I would rinse with some diluted bleach water. Let it dry in the sun for a while and just give it a good rinse of clean water before filling it again.
Re: How do you clean the kiddie pool?
My H used Dish Soap last time to clean our Inflatable pool. Not sure if he used a sponge (from washing the cars) or a brush but it worked.