Hi Sarah,
How exciting to be planning your first trip to Walt Disney World! I am glad that you are making your plans and considering how to move about the theme parks in advance. Some advance planning such as this is going to make your vacation smooth while you are there. You also have some great plans. My family dines at
Chefs de France at Epcot on nearly every trip. The food is delicious, and the ambiance is that of a bustling French bistro.
Mickey's Very Merry Christmas Party is so magical, with special entertainment and touches all evening. This sounds like a wonderful evening to me!
I do believe that you have left yourself enough time in between your dinner reservation and the party. When you are finished dining, you can walk to the front of Epcot and take the monorail from Epcot to Magic Kingdom. If you finish dinner by 5:30 or 6:00 p.m., this should give you enough time to be at Magic Kingdom by 7:00. The start time of the party is fluid, and while of course you want to be there and enjoy as much of the Christmas magic as you can, if you are a bit late, it is perfectly fine. Also, if you are a bit early, you will still be able to get into Magic Kingdom as party guests are typically allowed into the park within the window that you will arrive.
I hope that these thoughts are helpful for you, Sarah. Please do let us know if you have any further questions!