Connecting my apple id to an iphone 4

My iphone 4 that i am letting my child use will not let me log in, i enter in the username and password and then a notice comes up on the screen saying ; " an apple id verification code is required to sign in. Type your password followed By the verification code shown on your other devices " my problem is that i no longer use my iphone and am not able to get onto my iphone to access this code. I would really appreciate someone helping me with this.

fianna