It depends on just how deep in the sewer you're GOING, really. If you're going way down deep into the very bowels of the earth, then sure, but these are cats, looking for another cat - a witch's cat, sure, but still a cat, and cats have to stay where they can get at food sources, which is largely closer to the surface. (In a big city, this might be different, what with rat infestations everywhere, but it looks to me like this is set in the 'burbs somewhere - it probably doesn't HAVE much in the way of real deep-down sewers.) I wouldn't think they'd be going much deeper than storm drain-level - and at that level, you still have ambient light from manholes and gratings and whatnot.