I've tattooed older rabbits with no problems, and I get very little fading, but I also used to tattoo people, so I would say I have a bit of experience.
As the kits age and grow, the tattoos will 'grow' too. To make sure they don't fade, don't be afraid to tattoo deep. Well, if your using a gun that is. If your using a punch squeeze hard. Think of it this way, on a human, and I'm assuming a rabbit is probably the same, but you want to go through 3-4 of the 7 layers of skin to get a good, clean line that will stay. That's half way through our skin. You'll want to do the same for a rabbit.