I've never bred snakes but from what I've read from others it would be safe to say that whatever the breeder used for their adults, they used for their hatchlings as well. Aspen, cypress, newspaper, paper towels, etc.

Personally I use aspen for all my snakes. If I were to breed, I'd use aspen or paper towels. Aspen because it's what I'm accustomed to, paper towels cause it's easy and I've used it before for a new/young snake.