Welcome aboard, Kristi! The two adventures you mentioned are the perfect Port Adventures for families of all ages when traveling to the Eastern Caribbean!
Any Port Adventure with Disney that says "All Ages" and lists costs as $0 (ages 0-2) means that even the infants and toddlers are welcome to join in on these adventures with their family. And, luckily for you, there is no charge for those little ones! As long as the adventures you choose note that All Ages are welcome, you will only formally register those over age 2. Since your son is one year old, you will not be able to formally "add" him onto this Port Adventure, however, he is able to go along with you. Go ahead and register the rest of your family for this adventure and you'll be all set.
Both of the adventures you mentioned, the
Coral World/Butterfly Garden in St. Thomas/St. John, and the
French Riviera Beach Rendezvous in St. Maarten are for all ages. I do let guests know to be careful to note the ages when choosing port adventures as there isn't any wiggle room. Those adventures starting at age 5 are strict and unable to accommodate a child who is still 4 or under. So, the Family Adventures are the best to stick with when traveling with little ones.
I hope you have a great cruise on the Fantasy, Kristi!