Wishes is absolutely amazing! There are so many magical things I love about WDW - pixie dust fills the air everywhere. Most the of table service restaurants even outside the Magic Kingdom that offer a view for Wishes (California Grill at the Contemporary & Narcoossee's at the GF are great examples) dim the lights & pipe in the soundtrack.
Honestly, I don't think being inside Cinderella Castle gives you a good vantage point to watch Wishes. While you may be able to see some of the fireworks that are launched behind the Castle, you will not be able to see those launched above & to the sides.
If you're planning to dine there near the showing of Wishes, I would make an ADR early enough so that I could enjoy dinner & then head out the front entrance of the castle. From there, take a left & try to get a spot near the Rose Garden. That way, you'll get a great view of the fireworks & Tinker Bell will fly right over your head.
I know we've discussed best viewing areas for MSEP & Wishes but if you're looking for a special experience in the MK for Wishes, I highly recommend the Wishes Dessert Party - its AMAZING! I recently answered another question with details of this experience - click
here. To book a reservation call Disney Dining at 407-WDW-DINE. Pre-payment is required at the time of booking.
Prices are seasonal & usually range from $21.99 to $27.68 for ages 10 & up and $11.99 to $14.90 for ages 3 - 9. Magic Kingdom Theme Park admission is also required & is not included in the price of the dessert party.
Continuing to wish you many magical memories :-) Allison