No one in my family enjoys big "thrill rides." When my son was younger he was, at first, afraid of many of the rides at WDW. Here's how we helped him decide what he wanted to try (and encouraged him to try some he was hesitant about). We started off by explaining that Walt Disney created
Disneyland and Disney World so there was a place where kids and adults could all have fun together, the key word being "FUN." Walt's goal was not to REALLY scare anyone or make them feel yucky on a ride - hence the reason The Haunted Mansion isn't really all that scary. We told him that the rides all have some kind of storyline and that you have to look around at everything while you are riding so you can follow the story (that helped him not only be focused on when a big drop might happen). We let him stand outside for as long as he wanted to watch the rides that are visible (BTMRR, Splash Mountain, etc) and told him where the biggest drops are. We pointed out happy, smiling kids his age that were getting off the rides. He would then typically decide on his own to ride things and before any drops I would tell him to keep his eyes open (I think they feel worse when you close your eyes) and to take big breath right before the drop so he could scream as part of the "fun." Then we would make a big deal about laughing during and after any drops. He ended up loving every ride at MK, even if he did get a bit nervous right before the big drop on Splash Mountain. In fact, he ended up being a huge Space Mountain fan and we would wait outside while he rode it over and over (we don't like how jerky it is).
Now, I will say that after he figured out that the vast majority of rides he was concerned about were no big deal, he got a bit overly confident and thought he could handle Tower of Terror just fine. Personally, I don't like really big roller coasters but TT is my very favorite WDW ride. I find the drop fun and not at all scary although for years I was sure I would hate every second of the ride. My son - not so much. He did actually hate every second of it and we are glad he had already "conquered" all the other big rides at WDW because I think he would have been scared of them if he had ridden it before the others. Expedition Everest has never been his favorite ride but he does it. He always says the thought of the big drop scare him but, like many people, it happens so fast that you often don't even realize it's happening.
I agree with others about starting with Barnstormer, then 7DMT. Then I would do BTMRR, Splash Mountain, and finally Space Mountain. If it's just drops he doesn't like then you could move onto RNRC, there are no drops but you go pretty fast and corkscrew upside down. I hate big roller coasters because of the drops and I have no problem with RNRC at all, my son was fine the entire ride. EE can be a tough one for some and many hate TT. I found that if we had a positive attitude about the rides and made sure that he saw that we enjoyed the ride, he was just fine.
I think the idea is the same for your younger one. Be really positive and point out the storyline on the rides as they are happening, it will distract her if she is concerned about the dark on something or if she thinks something is a bit scary. When we were concerned that our son might find something scary when he was that age we would tell him there was a "silly" part coming up. It seemed to always work. Since you have already watched videos of the rides you will know if something is coming up that might be scary to her. We had our son giggling throughout Haunted Mansion at a very young age because we pointed out all kinds of detail when he was very young and focused on funny and silly things. He has never found HM scary in the least bit whereas I was TERRIFIED of HM until I was 10 because of what I THOUGHT it was going to be like. Then I had a friend point out that HM is more funny than scary and I got brave and tried it. Afterward I was bummed that I had missed out for so long thinking it would scare me.
