Diatomaceous earth and/or a borate product will probably do the trick:
How old are the kittens? Do you know for sure that they're feral? What is your SPCA's policy on ferals? Many just euthanize cats that aren't friendly enough for adoption - you might want to make sure that's not the case before turning them over. If the kittens are too old to be socialized, perhaps contacting an organization that will TNR them is a better idea.