Well, my situation was different because the animals were outdoors, but this summer I got an infestation of fleas from two feral dogs that we were trying to tame. (We couldn't - they're gone - that's another sad story. )

I was able to eradicate them by drawing a perimeter in boric acid around all water sources: the basement floor drain, the dehumidifier, and all the commodes that sweat condensation in warmer weather. Fleas can go a long time without eating, but like most critters, they need access to water.

Boric acid isn't completely non-toxic to mammals, but if you could put up gates or something for a few days, to keep your cats from walking through it and then licking their paws, basically as long as they don't ingest it or get a big lungful of powder when you're spreading it, it's safe enough. I'm at high risk for breast cancer (for one thing) so I'm pretty selective about what I'll expose myself to, and I was prepared to keep getting bitten five or six times a day vs. most flea treatments on the market, but boric acid did the trick.

Good luck.