Whether an 8:00 reservation will enable you to see fireworks from CRT will depend on what time Wishes is scheduled. Also, while it's an interesting view, you will only be able to see a limited number of the fireworks. The view from CRT is a different perspective, but not the best so I would only recommend it if you've already seen Wishes from Main Street/the hub/outside.
As others have said, only you know how your child will react to a late dinner. Often on vacation schedules and routines that are followed at home are changed. Some little ones adapt readily while others have a harder time.