Kits urinate on their own right from birth, usually on each other. Large litters can literally soak the wood chips under them. Kits sweat in the summer, and produce condensation in the winter, nestboxes can get get wet without the doe urinating on her kits. Wood chips make the box easier to clean, pull the fur out, dump the box, replace the wood chips and hay and remake the nest. I use wood nestboxes, not easy to sanitize them well. Always clean the nestbox out on the 7th, 8th or 9th day, before they open their eyes, it cuts down on nestbox eye infections. Hay quality does matter, rabbits are very sensitive to molds, dust and some toxic plants that can be found in poor quality hay. The kits will chew on and eat whatever you put in their nestbox, whether its straw, hay, newspaper etc, its going into their gut. I prefer to give them good quality grass hay to start the switch over from mothers milk to solid food. Granted I'm breeding for show rabbits, so my husbandry practices are a bit regimented after all these years, and I'm big on prevention. I really hate nestbox eye, haven't had a case in a few years now, figure its due to my being so picky about nestbox cleanliness. Your mileage may vary.