Found this info on another website:
Bed rails are available at no charge on request at all Disney resorts and the Disney Cruise Line, no matter which pricing or amenity level they’re on. In addition, bed rails may be requested for any guest who may need them, not just small children.
It’s a good idea to make your bed rail request (or any special requests, for that matter,) when you make your resort reservation, especially if you are planning on visiting during a busy time, as quantities are limited. If you have made your reservation, but not made your special request, call Disney reservations (1-407-939-6244) and have them add it to your existing reservation. If you booked through a travel agent, have them add your request for you.