Ahoy, Mark!
Thank you for coming to the Disney Parks Moms Panel to ask your question. It's great that you already have a plan in mind for your dinner arrangements.
Since the second seating for dinner starts at 8:15pm, I would try booking the
It's A Small World Nursery for 9:00pm. On your first night at dinner, you should definitely communicate your plans to your serving team. They will do their absolute best to accommodate your needs. In the 45 minutes you are together, the servers should be able to get your orders, drinks, appetizers, and your son's meal. Then, one of you should be able to leave for a short while to take your son to the nursery.
Since you will have communicated with your servers, they will know to not bring your meal until both adults are back seated at the table. If this does not work as smoothly as planned the first night, you could always try changing your nursery times slightly.
I hope this helps with your cruise planning! Have a wonderful voyage on the Disney Fantasy.
Happy Cruising, Tracy V.