Oh no! Poor itchy babies.

Fortunately I've never had to use any type of flea treatment, but I know people here use either Frontline or Advantage. And yes, I believe it is Hartz that is dangerous. Good luck getting rid of the critters 'cause from what I've heard, it's not easy because once they're on the cats they're in the house, so you might have to do the flea treatment for quite awhile to make sure they're gone for good.

Hugs and smoochies to both of your sweeties.