Lucky Dad!
Your plans sound great, but where you want to do your fireworks cruise, Magic Kingdom, or Epcot?
Here is what you need to do. Mark on your calendar whatever day is 90 days out from your first day at Walt Disney World. That is the day that you can call and reserve your cruise and meal. Also, you should be able to check on the website calendar the schedule for the fireworks.
On that day call 407.939.2329 to reserve your cruise. Also call for your Advance Dining Reservation for your meal. Allow yourself time to dine (40 min to 1 hour), time to travel to wherever you are picking up your cruise, and be there 30 minutes before fireworks time.
My suggestions for meals are the following;
Magic Kingdom:
Chef Mickey's in the Contemporary
Epcot: G
arden Grill in Epcot's Futureworld
Both are great, and conveniently located to get to your cruise. If, however, I had to choose, I would go with the Magic Kingdom/Chef Mickey's option.
Have a delicious vacation!