Welcome Aboard, Katrina!
It's always tough to know what to pack for a cruise, especially when you may need an item that's difficult to pack or travel with. I'm sorry to hear about your mom's back spasms, but hopefully I can help with your question regarding heating pads.
I spoke with the folks at Disney Cruise Line today and asked whether or not heating pads could be brought onboard. While they may have been prohibited in the past, I was told they
are allowed as long as the guest uses good care and turns them off and unplugs them when not in use. Basically, use the same judgement you would in your own home and make sure to use the heating pad in a safe way. If your stateroom host were to discover a heating pad left on while the stateroom is unoccupied, you might be asked to discontinue use of it while onboard.
Also note - on a Disney Cruise you'll have access to a wonderful state-of-the-art
spa facility with wonderful massage therapy treatments, steam rooms, and more. Your mom might enjoy pampering herself with a relaxing afternoon in the Rainforest Room or enjoying another amazing treatment. I encourage you to check out all they have to offer!
Hope this helps with your planning. No worries - your mom can rest easy knowing she'll have the comfort of her heating pad if she needs it while sailing! Please let us know if you have any further questions as you plan your vacation with Disney Cruise Line! - Diane W.