.....oooookay.....that doesn't mean they sold you two week old clowns. The fish would likely not have survived even being moved to your tank at 14 days. The store may have only had them for two weeks...but contrary to popular belief, creatures are not born the moment they arrive at the store.
Back to the topic at hand, the gentlest Ich treatment is hyposalinity, however I have no recollection of ever reading about it being used on fry. Either way you choose to go on this is going to be a gamble...if they're eating well for you, maybe the best course of action this time around is to wait and see...if they make it, you've got some tough little clowns...if not, well, it'll be a sad day.
I don't know what to suggest in avoiding this in the future, either...unless you move EVERYTHING out of the display tank and leave it fallow for a good month, it's pretty much always going to have ich in it, and you certainly don't want to disrupt the spawning pattern of the parents...hopefully someone else has dealt with similar issues in the past.
All I can say is best of luck...I hope they pull through.