Hi Alyssa!
So excited about your first trip! My daughter is 5 years old (she was 4 at the time of our last big vacation) so I know the magic you are about to see through her eyes!
Since it's your first trip it's really hard for me to narrow down your must-sees....because I LOVE EVERYTHING! But also, because really, there is so much that it kinda depends on who you are and what you like. One of my kids' favorite things to do is look through pictures online or in our Disney guide books (we have Brinbaum's for Kids and The Complete Walt Disney World). It gets them SO excited about the parks. PLUS it helps me gauge what they like and what they interested in and THOSE are the not-to-miss attractions!
BUT! Here are some of my daughter's favorite attractions from our vacation in September:
Animal Kingdom--Kilimanjaro Safari and Finding Nemo: The Musical
Magic Kingdom--Mad Tea Party and meet and greet with Princesses and Fairies
Epcot--Kidcot Stations and Spaceship Earth
Hollywood Studios--Disney Junior LIVE and Muppet Vision 3D
If you are looking for a weekly itinerary I suggest you check out TouringPlans.com because they show crowd predictions for the each day.
If you have more specific questions about your days or anything please
ask me directly! I'd love to help!