DD was 2 on her first trip. Based on on our experience I would say Peter Pan is a must do. DD is 9 now, so she was a toddler and pre-schooler pre-FP+. The only ride we FP at MK was Peter Pan. The minute we were able, we would just grab another FP for it. We would usually FP it 4 or 5 times during the day. It was the ride at MK she wanted to ride over and over. If you can get Anna & Elsa, that is probably another one, assuming she has an idea of who they are. DD was obsessed with Cinderella at age 2. Cinderella did not have a FP back then, but if she did, that would have definitely been a choice for us. Pooh or Enchanted Tales with Belle are other good options. Mickey Mouse would be another choice.
For DHS, definitely Toy Story Mania. The Frozen Sing Along would be another good one for that age. For Animal Kingdom, definitely Safari. For EPCOT- she will not be tall enough to ride the main 2 that FP+ is useful for- Soarin and Test Track. In September, the lines for the other stuff probably won't be that long- but the things she will probably like are Living with the Land, Nemo, Turtle Talk with Crush, and Journey into Imagination. DD's absolute favorite at that age was the boat ride inside the Mexico Pavillion (don't miss that one!), but I don't think it has FP+- and the line is rarely more than 10 minutes.