Hi.. a couple of things to think about:
1. Make sure the litter is clean at *all* times. Don't wait several days to clean it out. Some cats are so fussy they won't use it unless there are no solids in it at all. My cats are that way. If there is anything in the litter box, they'll stand outside and howl until I remove it.
2. Use an unscented type cat litter.. either clumping or plain clay.. a lot of cats don't like scented litter, or don't like the feel of the 'alternative' litters such as shredded paper, cedar chips or alfalfa pellets.
3. Make sure the litter box is in an out of the way area, never put food or a litter pan in an area that is high traffic. Make sure you don't have the cats food or water bowls near their litter pan. Don't allow anyone, including children to disturb or frighten the kittens when they are in the litter box.
4. When cleaning acccidents,do not use an ammonia based product. Urine contains ammonia..so if you use ammonia to clean the accident..the kitty will simply come back to the same spot over and over again.
5. If you catch the kitten in the act of urinating in an unacceptable area (i.e. outside of the litter box) pick it up (if you can <g>

and put it in the litter box. Don't scold for accidents.. lots of praise for using the litter box. Praise gets you a lot further than scolding. Contrary to what some folks believe, rubbing a pets nose in an accident will not cure the problem. If that line of thought worked, parents would be using it to potty train their kids.
6. If this behavior persists, call your veterinarian. Cats will sometimes refuse to use the litter box if they have a urinary tract infection. They seem to associate the litter box with the pain..so go elsewhere to urinate.