I'm so sorry to hear that your cats are sick. Yes, you can put them in the bathroom and make it steamy so they can breathe easier. If you have a vaporizer you can use this too. You can also buy Little Noses Pediatric Nose drops and try to put one drop in each nostril once a day to see if this will help.

I'd take them to the vet though because they may need more help to get over this. They may also become dehydrated and may need to have sub-q fluids depending on how bad they are. I hope that they feel better soon.