I'm also very photosensitive (sun and heat allergy). I would break out in red hives, which would feel very hot to the touch for hours. There were even times when I actually became ill (kinda felt like I had the flu, sick to my stomach, etc...) and that would last for a couple of days.
Recently, I learned from my physician that most allergic reactions, such as hives, is a histamine reaction in the body (of course I thought to myself "Well, no duh! Why didn't I think of that?"). My physician suggested that I take an over the counter, non-drowsy antihistamine, and then see how I do in the sun.
So, I went to
Walmart and bought a bottle of Equate 24 hour, non-drowsy allergy relief (it's Loratadine 10 mg) and started taking it this spring, thinking no way is this gonna help me. Well, you know what? It really DID HELP, and I mean A LOT!

For the first time in my life (and I'm 35 year old) I could go out in the sun, and not get hives or feel sick. I now take one every morning, and I'll continue to do so during the summer and early fall, while it's still quite sunny and warm out.
Of course, I still wear plenty of sunscreen to keep from getting burned, and I try to stay in the shade when possible, but now I don't panic if I HAVE to be out in the sun for a little while. I actually spent 5 days during the 4th of July week out on a lake, on a boat, in the sun practically everyday, and I had no problems!

It was FREEDOM!
I suggest you talk to your daughter's pediatrician and run the idea by him/her. It certainly helped me, and if your pediatrician thinks it's safe for your daughter, you should give it a try.
Hope you have a wonderful time at Disneyworld!
