Welcome aboard, Victoria! I bet you are down to counting the hours until your cruise at this point!
The shows on the Fantasy, and on all of the Disney ships, are absolutely fantastic! You'll be treated to
Broadway-quality shows almost every night, with some special guest acts on the other nights. Each evening, there will be two show times, and sometimes a late-afternoon third show added in too. If you have the Second Seating for dinner, your best bet is to take your family to the 6:30 show in the Walt Disney Theatre. The shows run 45 minutes to an hour at most, so you'll have plenty of time after the show to wander a bit before you need to head to the restaurant.
Schedules and show times are always subject to change, but you'll find that they're usually right around 6:30 and 8:30 with the extra show at 3:00 sometimes.
I think you'll be surprised at how well your girls will adjust to staying up later on the ship! My kids who are always in bed by 8:00 p.m. at home have been known to still be going strong until midnight in the Oceaneer's Club, as they're having so much fun! A nice afternoon nap each day will do wonders for them (for adults too!).
If your girls like the Club and Lab, they might want to take advantage of the Dine and Play option with the Second Seating. Your family dines together, but the kids meals are brought out first. Once they are done, about 45 minutes into your seating, counselors from the clubs meet at the front of each of the main dining restaurants, where they will take the kids up to the clubs while the parents enjoy a quiet remainder of their meals. My kids love to do this, and my husband and I like to spend the second half of the meal catching up with each other, just the two of us. It's a great option!
Have a great cruise next week, Victoria!