Hi Deonie,
If you have confirmed the Magic Kingdom opening at 10:00 am, an 8:40 am reservation at Crystal Palace should allow you enough time to finish eating before the parks opens. However, I'm worried that you may have misread the hours since typically the Magic Kingdom opens at 9:00 am; on Extra Magic Hour days it opens at 8:00 am for resort guests. If you click
here, you can confirm the hours. If the park actually opens at 9:00 am, I'd try to make an 8:05 reservation, which is usually the first available time.
If you are finished eating and ready to go before the park opens, you won't be able to go very far, though, since the sidewalks into the "lands" will be roped off until the park officially opens. However, you will still be ahead of the crowds and have already had a morning with characters (no standing in line later to meet Pooh and friends!). You will also have the chance to get some wonderful photos on Main Street USA with the castle in the background with very few other guests around.
Thank you for visiting the Moms Panel; please let us know if we can help again.