soak the toys in a hot soapy bleach solution, use a scrubber for those hard to reach places (remember to wear gloves), rinse well, then run through the dishwasher if possible, (take out before the dry cycle), and buy a tub net to allow the toys to dry out completely between bathings
Chlorine bleach diluted to 1 part bleach to 10 parts water is a good, fairly non-toxic everything-killer. Mold, bacteria, viruses... bleach kills 'most everything. More important than bleach for mold prevention is making sure that bath toys get a chance to dry out real well. Store them in a dry, well-ventilated place between baths.
You can spray down the bath toys with special mold resistant cleaners bought from the store, then rinse them well. Easier and cheaper, before you drain the tub all the way, pour in a little bleach and swish them, then drain them and rinse very well. Don't forget to empty any toys that hold water.
Clean frequently so mold doesn't have a chance to grow. You can use either a vinegar and water solution, denture tablets, or even a mild bleach solution for especially dirty toys. Soak for several hours/overnight (or 15 minutes if using bleach), making sure that the insides are rinsed out. Scrub with a bristle brush and rinse th
Plain, ordinary bleach is the way to go. Use a very light dilution, about 1-2 teaspoons per gallon of water. Let them soak for about 30 minutes and rinse thoroughly. Best way to do this is in a kitchen sink, may as well get some of your kid's other toys and throw them in there too!