Its not a good idea to have one. usually children especially girls are highly suggestible and may disclose the password in confronting situations. Also they are vulnerable to emotional blackmailing. Its also easy to distract a child.
What if someone tells that "your mom told me the password, but it didn't stay in my mind, as i frantically drove to pick you up after admitting her in the hospital".
The best way is to
1.Make sure that you or any other from your family will be available for picking her up or what ever (its better late than never)
2.Make her memorize your mobile, office and address
3.Teach her to make use of public communication including telephone
4.Teach her to get the help of law enforcers, police in any emergency
5.Never get directions from unfamiliar people
6.Train assertiveness to your child especially when to say no
7.Give life skills training to your child
This will also help her to face emergencies like being lost in a crowd