Hi.

I would also say do not feel guilty, and that happens some children or families are just not the right fit.

When you show that you're worried "which is posted in your comment", that means that you're stressed with the situation. Explain that to them. Be honest and tell them that it is affecting your well being (very important for doing this job), and because of your commitment to other families as well, you had to take this irrevocable desicion. Wishing for them to find a right fit and support for their child and them.