Honestly, I don’t know what cat litter is best for kittens! But I can think of six things which may be useful to know about. At about 5 to 6 weeks of age kittens can begin using a litter box.

Lifelong preference?
The type of litter substrate that a kitten is given to use and likes might be preferred by an individual kitten for the rest of their lives. Therefore it is quite important to select one which is the safest.
Dust
Depending on the product, I’ve always thought that a lot of dust can be kicked up by kittens and cats when they use their litter tray (digging around and burying poop). It is obviously unhealthy to inhale this dust. Kittens are smaller and therefore closer to the litter which is why dust particularly concerns me. Some litters are particularly dusty and the dust particularly harmful. I can start you off on that route by referring to Tidy Cats litter. Read about it by clicking on this link. Don’t buy it. I would find a litter substrate which has a very low level of dust. I’d do some research on that.
Non-clumping
On the topic of minimising dust, it may be wise to purchase a non-clumping litter because if some dust is inhaled it won’t clump and cause harm. I think that is probably the biggest issue to think about together with how it feels underfoot.
Softer litter – smaller particles
Kittens’ paws are more delicate. The litter substrate should not irritate their feet. It should be softer if possible. Once again I think it requires personal research.
Adults and kittens have the same or similar requirements
The point is though that people do have ideas about specific products which are good for kittens but you don’t find Jackson Galaxy, the American cat behaviourist specifying a cat litter which is ideal for kittens. He discusses what makes a cat litter effective in general terms, both for adult cats and kittens.
Getting into the tray/box
Another point worth mentioning, perhaps, is that because kittens are very small the sides of the litter tray should not present a barrier to a kitten and the same goes for seniors who have lost their mobility and obese and disabled cats. Puppy litter boxes are pretty good for senior cats and cats with mobility-based pain or discomfort. The same might go for kittens.
Wood-based?
Personally, I’ve always favoured a wood base litter and if you get one which is fine rather than pellets (which may be uncomfortable for a kitten), perhaps that might be the way to go. Environmental issues are also a factor in selection. I raised my kitten using wood-based litter and it was standard pellets. He was fine with it. No dust issues.
Conclusion
The bottom line is that there’s not a lot of difference between a kitten and a cat when it comes to cat litter and therefore the best cat litter for adults might well be the best cat litter for kittens as well. The problem is that you have to wade through a plethora of advice and advertising on the Internet to find the best products and there won’t be a single best product either. Dust must be the biggest issue in my view. Avoid dust at all costs.