Yes, Flip works great for kittens. They will be able to use a litter box at around eight weeks and to help them begin, just do the following.

Open the Flip lid and place them inside. Gently motion their paws through the litter so they become familiar with how it feels on their paws and how it smells. It should take approximately a week for them to begin to understand it, and to know what to do on their own.