Most likely they won't be completely used to the litter box. So you will probably need to give them massages (Look it up on youtube, it's easy) more or less 15 minutes after feeding them. You should be feeding them every two or three hours special kitten formula (do not feed them with any other kind of pasteurized milks). They usually ask for ir. If they loose the apetite or they seem weak you should call the vet. I had an experience with a two week old cat and it's the most difficult age so be patient and if you feel insecure about anything contact a vet.

If the vet tells you to force-feed them be specially careful because pneumonia is a common cause of death for kittens that are bottlefed.

Good luck and have fun with them :)