Hi Adam John,
From memory, I do not recall there being an option to request bed rails as part of the
Online Check-In process.
All Disney Resort hotels do however have bed rails available upon request at no additional cost.
In order to request a bed rail prior to your arrival at the Walt Disney World Resort, I would recommend that you contact the hotel direct to have your request added to your reservation. Alternatively, you can request one when you check-in at your Disney Resort hotel.
Looking at the previous questions which you have submitted, I believe that you will be staying at
Disney'sAll-Star Music Resort. You can contact this hotel direct by calling 001 (407) 939-6000 from here in the UK. (I would suggest calling via Skype in order to keep costs down to a minimum.)
Wishing you a magical holiday filled with memories which will last a lifetime!
Emma