Ahoy, Olenka!
Congratulations on your third little cruiser! How exciting that you are expecting AND planning a Disney Cruise Line voyage on the amazing Disney Fantasy. I have some good news and some bad news. First, the good news is that your cruise is far enough out that moving it should not be a problem. The bad news is that you will need to move your cruise to begin at least after May 6, 2016.
According to the Disney Cruise Line website, infants must be at least
6 months of age on the day of sailing in order to be allowed onto the cruise ship. Since you are currently booked for a Western Caribbean 7-Night sailing on April 30, 2016, you can book the exact same itinerary for two weeks later, on May 14, 2016. When I checked tonight, the sailings appeared to be similar in price.
I hope this helps with planning your cruise on the Fantasy! Thank you for visiting the Disney Parks Moms Panel! I look forward to answering more of your questions as your sailing gets closer.
Happy Cruising, Tracy V.