You will not be wasting a trip at all! We took our sons when they were 11 months and 19 months (2 different trips) and will go back with a 23 month old. The look on their faces as they met the character was priceless! They also both loved the rides and the fireworks. I like getting to the parks for early EMH so we can get a lot done in the extra hour. We went back to the hotel in the middle of the day if we needed the break, but both boys were able to get good naps in the parks...we played that by ear. Plan the trip around their schedule so you don't have meltdowns. We only had 1 "bad" day on the 3rd day of the trip because DS was overtired and overwhelmed.
Use the baby care centers to take a break and have a feeding if needed. For a fun experience, try to hold off on the 1st haircut until you get there. If they need a cut, get it done at the Harmony Barbershop in MK. They will get a certificate and a free set of ears that says first haircut.
Have a great trip!