Hi Shirley!
Thanks for visiting the Moms Panel with your great question!
You'll have no problem at all reaching Disney's Polynesian Resort for your dinner reservation at 'Ohana.
Although Magic Kingdom may be closing to regular park Guests at 7pm,
Mickey's Very Merry Christmas Party, a separately ticketed special event, takes place from 7pm until midnight. You'll find that plenty of buses will be operating from All-Star Music to Disney's Magic Kingdom Park throughout the evening for the party. Upon arrival at Magic Kingdom, simply jump on the Resort Monorail or take a Resort Launch for a pleasant boat ride across the Seven Seas Lagoon to your dinner reservation.
Be sure to allow for a full hour of travel time from your resort to
'Ohana.
Following your dinner, you'll find that you'll be able to take the Monorail back to Magic Kingdom and board a bus right back to your All-Star Music Resort, since buses from Magic Kingdom will be operating well past midnight that evening!
I hope you'll write me back again soon with more great questions about your upcoming vacation.
Wishing you a simply Magical time at Walt Disney World Shirley!
Kirsten