It is very difficult to hand our babies over to other people. Even the ones my kids took!
May I make a few suggestions? You can sign a contract with them; you can stipulate that if for any reason they can't keep the kitten(s) that they come back to you; you can ask to do a home visit, both before and after the adoption; you can request the cat not be declawed, remains an indoor only cat, etc; you can, and should, charge for the kittens. Even if it's only a nominal amount. That lessens the chance that the people are taking them for snake food or science labs. You are perfectly within your right to do all of that.
This is a great sample contract that you can change to suit your own purposes.
All of this does not make it any easier but it does help to ease some of the doubts. It is still within your right to tell the adoptee you have changed your mind about them adopting if you feel something is not right.