Just a thought here. The back porch may not be too unsafe if you can put something up to block off the stairs. Momma can get over and kittens won't been able to for sometime to come. Seeing they are new borns they won't be wandering for awhile yet but atleast with something blocking the stairs they won't be able to get hurt if they fall when they do start wandering about. I would suggest a newer box with something inside and maybe momma will take it into her head to move the babies on her own.