Hello, Amanda!
Twelve days in April...what a wonderful trip you have in store!
We were just at the Magic Kingdom, and noticed that Disney has, as usual, worked a little magic in keeping the outskirts of Fantasyland as unaffected as possible by the nearby expansion. It seemed as if the Imagineers did their best to keep everything in Fantasyland up and running while that enormous expansion on the other side of the walls continues.
Regarding what Disney calls "Operational Updates," you can find specific information based on the dates of your trip. Go to
Magic Kingdom Park's page within Disneyworld.com and you'll be able to click on each date on which you plan to tour the Magic Kingdom. Operational Updates are listed on the right side, once you've clicked on a date. I just glanced through a few dates in April, just to see what's listed for you, and didn't see any Fantasyland attractions which are planned to be closed at that time. However, Big Thunder Mountain Railroad is set to be closed from 01/09/12 through 05/27/12.
Be sure to check back from time to time to see if any changes have been made to these plans, however. Disney updates that site frequently, especially helpful if renovations move along a little more quickly than originally planned.
Enjoy your stay! Please come back with any additional questions about this, or another, visit to Walt Disney World!