Hi Jessica,
Thank you so much for coming to the Disney Parks Moms Panel with your question! You are choosing between two fun and delicious restaurants for dinner.
'Ohana at Disney's Polynesian Village Resort features an all-you-care-to-eat Polynesian feast, as well as games and dancing.
Whispering Canyon Cafe at Disney's Wilderness Lodge is a jovial environment including sing-alongs, hobby horse races, and an all-you-care-to-eat cowboy-inspired dinner.
In my experience, a meal at 'Ohana and a meal at Whispering Canyon Cafe take about the same amount of time. Both of the resorts that the restaurants are located at offer boat access to Magic Kingdom which is typically fairly quick. Disney's Polynesian Village Resort also offers monorail access to Magic Kingdom. While I think that you would likely be able to make it to the Magic Kingdom for a 9:00 p.m. showing of the
Main Street Electrical Parade after a 6:40 p.m. reservation at 'Ohana, you might be fairly rushed, and also might have an issue finding a prime spot from which to view the parade since others will find their spots earlier. If you can move your reservation earlier, or secure one at Whispering Canyon Cafe earlier, perhaps at around 6:00 p.m., I think you will have a more relaxed evening.
I hope that these thoughts are helpful for you, Jessica! Please do let us know if you have any more questions while planning your magical vacation.