Ciprodex if you have tubes like my son but it's like $140 at CvS. For normal people cortisporin otic is cheap. For mild,acetic acid rinses. For prevention, alcohol or acetic acid rinses after exposures. At least that's what i read on the Internet, no medical advice here
Since we aren't giving medical advice here, let me also let you know that, from the internet, the correct ratio of alcohol (rubbing alcohol usually although everclear works too) and acetic acid (White vinegar) is 1:1. It's pretty much the same thing that you find in the pharmacy (Swimmers Ear, Auro-Dry, etc.). Put a couple of drops in each ear after your kid swims and it should prevent the problem.