follow the link in my signature to the disABILITIES FAQs. Posts 18-22 of that thread are about access to attractions and list which may be difficult to step into and out of.
You may also want yo look at post 2, which is about renting mobility devices. At her age, I would not suggest an
ECV, especially since it is her first trip and she will want to be looking around at the sights. It takes a lot of attention and observation to drive an ECV. A wheelchair for someone to push her would give her less independence, but more ability to look at everything. A wheelchair would also give her a place to sit for the shows where you otherwise have to stand to wait for next show. All attractions are wheelchair accessible to the point of the theater or boarding.
I made some notes for you, but there is a lot more info in the disABILITIES FAQs thread.
Definitely:
Tiki room - show; without mobility device, guests have to stand to wait for the next show.
Riverboat - there are some benches on the boat
Hall of presidents - show; stand to wait for next show if no wheelchair
Small world - there is a wheelchair accessible boat. Without a wheelchair, it is a step over the side of the boat and then down into the boat
Philarmagic - theater; stand to wait for next show if no wheelchair
Carousel of progress - theater
I would say these depend on her interests more than anything else.
Aladdin - ride that elevates and goes round and around. If she gets dizzy, she might not like it. If you have small kids who will be riding, she may prefer to watch them.
Jungle cruise - this is a classic and is cute with very corny jokes. If it's hot, be aware that the ride is pretty long and is all outside. There is a wheelchair accessible boat. Otherwise, it is 2 long steps down. This is one where the CMs can offer a steadying hand (but not more)
Country bear jamboree - show. Corny country music. Some people like it and feel it's better skipped
Peter Pan - cute, elevated ride in a Pirate Ship. The moving walkway can't be stopped and you have a limited time to get on and off. If she is familiar with Pter Pan, she may like it. Since it's only about 90 seconds long, some people feel barely settled before it is over.
Pooh - it's cute ,but whether it's a go or a miss really depends on if she knows/likes Pooh
Teacups - I would not recommend. Can make people quite dizzy and unsteady, so doesn't mix well with mobility issues
Dumbo - same comments Aladdin except that Dumbo is harder to get into because the seat is very low
Stitch - only if she knows and likes Stitch. It's not a thrill ride and doesn't move at all. It's very dark and many people feel it is unpleasant
Belle - if you have little kids with you, it is cute. Especially if they will want to participate
Under the sea - has a moving walkway that can be slowed. It's cute, but best of you know the Little Mermaid story
Pirates - a classic. The boat is 2 long steps down to get in. Getting out can be more difficult because it is low. It's helpful to have one of your group get into the boat right in front of her to help her from inside the boat and another get on after her to help from outside. Same getting out.
Haunted mansion - guests can bring a wheelchair thru the stretching room all the way to boarding. Those guests board at the unload area; if you need the moving walkway slowed or stopped mention it there