CP will insist that cats are kept separate from yours, do you have a spare room that kittens could be kept in? It is much better isolation than a cage, as you dont know what kittens will have come into contact with, and they will have more space than in a cage. I have fostered for nearly 6 years and it is very rewarding, I have introduced some fosters to my cats, but my girls live upstairs through choice, so dont know/care who is downstairs, and show no interest in teh cat room, whereas some cats will really bother with new smells in one room, so an outdoor pen would be better in that situation. The hardest thing with fostering is letting them go, but fortunately, I have never struggled with that, as you are letting them go to a fantastic home, with more time/space than I can give, and it also frees up space for another needy cat.
Helen, most spare rooms are bigger than pens in rescues, and most cats are fine with it.