Hi, Shannon!
Thank you for bringing us your question. I hope you are getting excited for your upcoming vacation, it's going to be here before you know it. You're also visiting during the most magical, beautiful time of yEAR.
To modify your advanced dining reservation to include your nephew, you'll need to contact Disney Dining at
(407) WDW-DINE. By adding your nephew and increasing the number of guests in your dining party, availability will need to be confirmed. While you are able to update the names of the guests in your party listed on the reservation via My Disney Experience, you are not able to add more guests onto the reservation.
To expedite your request, make sure you have your dining reservation number handy. It's generally not a problem to accommodate this type of request, especially since you're adding a little one, but call in as soon as you can to increase the possibility of being able to add him - the biggest problem with reservation modifications occur if adding a guests changes the table-size needed to accommodate your addition.
I hope this helps, Shannon! Please do not hesitate to reach out to me directly if you should have any additional questions.
Faith