My firstborn's first trip was at 15 months of age... (I"ve also brought subsequent children at ages, 3 months, 4 months, 2 years, 3 years, 4 years, 5 years...etc. and here's my advice...
Get the hopper pass... that way you can plan on getting to MK for EMH and then head out for lunch around 1PM when the crowds are crazy... go back to the hotel...let her take a nap...then head to another park, like EPCOT for dinner and a few rides and fireworks... Also, Animal Kingdom is a park best to do early in the morning... take her on the Safari ride and do the animal trail.. then take her to see the Lion King Show and to meet the characters... then take her over to Dino Land and let her ride the Dumbo-like dinosaur ride there... then go to the Boneyard, UPSTAIRS and let her sit in the giant sandbox to dig and relax while you sit for a few under the shady covering that makes this one of the few cooler spots at AK.... Have lunch inside at Rainforest Cafe....or outside at Flametree BBQ by the water... then head out... let her have a nap back a the hotel... and then go to MGM for dinner and rides like The Great Movie Ride and the Back Lot tour... let her play at Honey I Shrunk the Kids play area (there are fountains and slides she'll enjoy)... Not sure how late the shows for Voyage of the Little Mermaid or the PlayHouse Disney Shows run--but she'd love those... she may be too young to sit through the stage production of Beauty & The Beast... You're call!
But you see....it's nice to go to a park EARLY.... and leave...relax at the hotel (maybe take her for a dip in the pool) to recharge everyone's batteries...then go back out again... Park Hopping, IMO, works really well with little ones....