Hi Andrea! What a fun evening you've planned! You should have plenty of time to eat dinner with Mickey and friends before heading over to the
Tomorrowland Terrace Fireworks Dessert Party. (I'm more worried about you having a big enough appetite to enjoy both of the buffets at Chef Mickey's and the Dessert Party -- you'll need to eat light that day so you'll be super hungry!)
Plan to arrive a few minutes early for your Chef Mickey's reservation so that you'll be sure to be seated by 7 p.m. Since the meal is a buffet, you won't have to wait for your food to be delivered, and the characters should all visit your table within an hour, or less, so you'll likely finish your meal around 8 p.m. After dinner, you can walk or take the monorail over to the Magic Kingdom for the Dessert Party. Since the monorail makes a few stops before arriving at the park, I'd recommend walking to the Magic Kingdom. (Walking takes about 10 minutes while the monorail could take 20.) Once you arrive at the park, it will take 15 minutes or so to proceed through the bag inspection, enter the park and stroll to the Tomorrowland Terrace, so you should arrive in plenty of time to enjoy some desserts while you watch Wishes.
Come visit us again at the Moms Panel if you have more questions! We'd love to help!