Hello Suzanne!
First of all, your family is going to LOVE
The Pirates and Pals Fireworks Voyage! My family experienced this swashbuckling adventure last year, and it was incredible!
For dinner, I would suggest eating a Quick Service meal at Magic Kingdom. That way you will be able to save some room for the yummy pirate-themed snacks and desserts provided before your Fireworks Voyage begins!
The Pirates and Pals Fireworks Voyage begins 90 minutes before the scheduled Wishes Nighttime Spectacular fireworks at Magic Kingdom. With small children, I would leave Magic Kingdom at least 40 minutes before the voyage begins. I believe this will give you enough time to take a bathroom break and get settled before the fun begins. If you have a stroller, it might be faster to walk to Disney's Contemporary Resort than waiting for the monorail. My wife and I did this with five kids (with two strollers), and it was not bad at all.
If you have other questions, please let me know!
Have a MAGICAL day!
Wes