I've never been in your shoes directly Lilith, and only you will know how to handle it best when the moment is upon you. I often find that sometimes saying thank you as I say good-bye makes everyone feel a bit better about the situation. Let the babies know that you are thankful for the time you spent with them and now you are more prepared for all the other babies you are going to cross in the future. Let them know how important they have been to your learning and that you hope that they found the time and lessons to be just as important and rewarding.