Letting them sun should help get the current stains out. Also, what is your wash routine? You should be starting with a cold rinse to keep stains from setting. Also, when they get poopy, rinse them off and then keep them wet in your diaper pail (not that the diaper pail has to be filled with water, just that you don't wring your dipes out after you rinse them). This will also help stains from setting.
Mommy to two super cute kids.