Hi Belinda!
You can book these reservations 180 in advance. Since this window has already opened for your vacation dates, I would suggest calling to book your appointments as soon as you know which day you will be visiting the Magic Kingdom and would like to have this experience.
I can understand not wanting to be tied down to reservation times. When we have to be a specific place at a certain time, I try to enjoy the attractions in that area beforehand so we won't have to spend a lot of time walking to get to our reservation in time. Before the Pirates League, you could visit Big Thunder Mountain, Splash Mountain, Pirates of the Caribbean, the Jungle Cruise, etc. If you're already on that side of the park then you'll only have to walk a few minutes to get to your appointment.
I personally think that your boys would really enjoy the experience. They will likely get added attention from the cast members throughout the day and would be something new and fun to try. After their transformation, try to catch the Jack Sparrow Pirate Tutorial show which is held next to the entrance of Pirates of the Caribbean. Jack Sparrow chooses members of the audience to participate and on my last trip lots of the boys (and girls) had visited the
Pirates League. You can call 407-WDW-CREW (407-939-2739) to make reservations.
I hope this helps! Have a great vacation!