Thanks for all the suggestions, everyone! I love hearing about others' experiences at WDW, and I read each post thinking, "Yes, that's what we should do." Ha, it's hard to decide! But hopefully we'll get to do it all even though there will be only one "first" attraction.
I love Philharmagic, Peter Pan, and Voyage of the Little Mermaid. But I'm quite sentimental about the films. Baby cousin is excited to meet the Princesses at Akershus and certainly enjoys Disney movies, but not as much of a sap as me about it. I feel certain she's going to be all about the thrill rides (and I love them too, thankfully).
I don't know if we'll have a morning at MK until our departure day. I hope we're able to ride a decent amount of attractions on our arrival night (we should get to the park around 4 PM) as long as we use our three FP+ wisely and stay late enough for the crowds to thin out. Based on crowd calendars and our reservations, right now I have this tentative schedule:
Day 1: Arrive, MK evening
Day 2: DHS morning and 50s Prime Time lunch, break, Epcot WS night
Day 3: Epcot Akershus breakfast and Future World, break, DTD or park of her choice at night
Day 4: free day to sleep in and go wherever during the day, Sci-Fi early dinner at 4 PM and SWW stuff at DHS at night
Day 5: AK EMH morning, whatever else we want for the rest of the day
Day 6: MK morning, BOG lunch, leave to drive home early afternoon