I think you should hand it to her personally and say to her I have thought long and hard about this and I feel I have no other option but I will no longer be able to provide care for your child everything is in the letter please read it when you get home and if you have any questions please feel free to call me. I wouldn't want a parent to slip a letter into the child's bag and not say anything to me if they were the ones terminating. Make sure you keep a copy for yourself.