Please help! iphone 4 not accepting passcode.

I have two iphone 4. one is old and I bought new one. I transfer the sim from the old to the new. Both using the same accout, the same passcode. Now the old one after there was no more sim, my child is using it, and after month suddenly it is not accepting the passcode. I need to open it to see my old tel number and the disccussion of the messages. I know my apple id and password so I went to itunes but when I tried to restore it, it will say turn off the "find my iphone app" how can I turn off when I can't open the phone.