The kittens' main source of warmth will be their mother. You can, however, line the box or cat carrier with a towel which can be soiled and washed.
[header] How to monitor newborn kitten growth [title] Give the kittens a warm bed. [step] During the first two weeks of their lives, kittens are helpless: their eyes will still be shut, and they should spend about 90% of their time sleeping and eating. As long as the mother cat is feeding the kittens, your involvement will be minimal.