Try Composure Chews; someone else on here had success with them. You can get them off Amazon.com

If the kittens are feral, I'm not sure how you would ensure each kitty gets ONE, so think about that. Here are 2 links with info on them:

http://www.vetriscience.com/composur...dogs-MD-LD.php

http://www.vetriscience.com/sellshee...re%20Chews.pdf