Hi, Daphne!
As long as your reservation at
Be Our Guest Restaurant is before Magic Kingdom closes, you should be just fine. While Guests with tickets to Mickey's Very Merry Christmas Party are allowed entry into the park a little early, Guests who are enjoying their day in Magic Kingdom may stay until park close. Once you have enjoyed your delicious meal, simply exit the park; no worries!
We've dined at other WDW restaurants closer to park close, and have finished our meals well after the parks have closed, with no worries. The only difference here is that you'll be exiting the restaurant into a specially-ticketed event, rather than an empty (and so beautifully lit) park. Because of this, Cast Members will assist you in exiting at that point.
Enjoy your visit to WDW during the most spectacular time of the year, Daphne! The parks will be in their holiday finest, the resorts will simply glisten with extra magic and you are sure to be wowed by Be Our Guest Restaurant. Enjoy!
Send more questions as you think of them; we love to help!