We just got back and took our 5.5 year old DD on FOP. She is a brave rider as far as roller coasters go but she is somewhat afraid of the dark. The ride she was most concerned about was the Haunted Mansion. She did it once and was proud of herself but didn't want to do it again. She LOVED Expedition Everest though, so she can handle rides obviously. She is somewhat small for her age but not teeny tiny. She was maybe an inch over the 44 minimum.
Anyway, this was my first time on FOP too so I wasn't able to prepare her as well as I could for rides I'm familiar with and I was afraid there would be some element of darkness that would scare her. There wasn't. All the buildup was a little much for her. She didn't exactly the get the whole parasite thing and the avatar matchup and all that takes so long that she was starting to get apprehensive but she handled it and we moved on.
In the end, she absolutely loved the ride. BUT two things I hadn't thought of were: 1. The glasses were enormous and she couldn't keep them on. We didn't have that problem with other 3D shows but you're not really moving around much in those. I'm not sure if there's something you could do to help with that. Maybe a hair tie or something? But it kind of bummed her out. And 2. She complained that the ride hurt her chest. I guess because it hit her in a weird spot being so short? I don't know.
Anyway, those were two things I never considered preparing her for. Or the amount of time spent in the preshow.
Otherwise, best ride ever! For us grownups and for her!