Welcome aboard, Michelle! Congratulations on your upcoming first sailing with Disney Cruise Line! This trip will be a very magical one for your whole family. I love to see the excitement as a first-time cruise family discovers all the amazing experiences Disney has to offer at sea!
Atlantis is a beautiful, one-of-a-kind resort with so much to offer families, so I'm glad you've chosen to enjoy a day there when you stop in Nassau. You are unable to select your son for this adventure since there is no cost for him, so just go ahead and select all the rest of your family and he can just come along, no ticket needed. This is because he falls into the infant price range for ages 0-2 on this particular adventure.
However, if you choose another Port Adventure during your cruise, just be sure that it is one that includes children his age. This will be noted in the price ranges at the top of the adventure description. Some adventures start at say, 5 years old and up, so an adventure like this would not allow your 2-year old, and you wouldn't be able to add him online or bring him with you. But, since the
Atlantis Beach Day and Discover Atlantis adventure does allow all ages, and notes the infant price of $0, you're all set!
Happy planning, Michelle. Please let me know if I can help with any other questions before you set sail next month!