Plain clay litter is horrible for odor control and tends to be very dusty. I highly recommend World's best Cat Litter for kittens, if they are older kittens the cheaper clumping clay is fine - I like Dr. Elsey's PRecious Cat ULtra or Scoop Away Unscented (still has a light scent but not as bad as some). Stay away from scented litter as they offend kitty noses and may cause litterbox avoidance problems.
If your kittens' feces smell terrible, you should also examine their diet and make sure they don't have intestinal parasites. A stool sample can be examined by the vet for parasites. To reduce stool odor always feed a high quality canned meat diet. Cats can't digest plant matter so it ends up in their stool. A high quality diet is more digestible and results in less stool output. It may cost more but the cats will be able to eat less of it and in the long run they will be much healthier.
Dry cat food not only causes stinky large stools, it leads to obestiy, diabetes and kidney disease because cats can't turn starch into energy - instead it is coverted into fat. They also cannot regulate their blood sugar well when fed carbohydrates since they lack one of the metabolic pathways to metabolize carbohydrates, leading to diabetes and obesity.
A raw meat diet is actually the healthiest but one step at a time
Here are some helpful websites with regards to choosing the best food for your kittens:http://catinfo.org/commercialcannedfoods.htm
Nature's Variety raw frozen or Instinct grain-free canned foods are some of the best:http://www.naturesvariety.com
Good luck with your new kittens!!