I would definitely recommend taking them to the vet, just in case. But has there been any major changes in the household (e.g. new furniture, new skin kid). Some cats are particularily "sensitive" to changes in their environment.

I have used Nature's Miracle before, but I have also found that Woollite produces a pet stain remover that works quite well (and is available at your local Walmart). Try and keep them away from the area until the soluntion dries and the smell is gone.