The second you place an advertisement seeking a nanny, you'll start receiving applications. It's not likely that a strong wind will blow them all away, leaving you with only one Mary Poppins. You're going to have to deal with telling most of the applicants they didn't get the job. We all hate sharing bad news, so it's tempting to just not respond to the people you didn't choose, but you know that you'd want a response, so pony up and get it done.
Develop a form letter that fits your situation. This doesn't have to be long. It can simply be something such as, "Thank you for applying for the nanny position. We received more than 50 applications. I regret to inform you that we've decided to go with someone else." You may also want to wish them well with their job search.
Add a personal message if you want. Some lawyers advise against giving details, according to GlassDoor.com, but if you called the person in for an interview or more, you may feel that the person deserves more than a form letter. You should still avoid specifics, but you might say, "I enjoyed meeting you and your love of children is obvious. Unfortunately, we went with someone we felt was a better fit."
Send the message promptly after your chosen applicant has accepted the position. Don't leave the applicants hanging. It's fine to send your notice by email or through the postal mail. You may also call candidates, although this may be the most uncomfortable way for you to share the news with them.
Some applicants may respond back, asking for feedback about why you didn't select them. Use your discretion in replying back to these people. You probably don't have the time to provide detailed responses to everyone, and it may not be in your best interest. It's usually best to be somewhat vague, stating that you chose someone else because that person had more experience or something more subjective, such as "similar ideas about discipline."
Avoid getting phone calls from applicants by sending out your rejection emails or letters as soon as you have made your decision. Remember that if you were the one seeing a job, you would want to know as soon as possible.