Hey Suzi!
I totally think that Mickey's Not So Scary Halloween Party is worth it!
During dinner you'll be sitting in one place all that time and you'll be ready to walk off some of that food. if you stay later to enjoy the party you'll get to see the Magic Kingdom in a totally unique way. My family was amazed at how much the Park transformed. We almost didn't go because we had had a full day, but I'm so glad that we did as we all had an amazing time.
Also, your daughter gets bragging rights for the next year because she'll get to trick-or-treat in Disney World. A lot of kids come dressed up for the Party so it'll be a good chance for her to show off the BBB magic.
My little girl is also 7 and experienced MNSSHP last year for the first time and brings it up at least once a month...she can't wait to go again this year. I think they're a good age for this experience.
So, yes, I would take it leisurely that day at Downtown Disney (oh my gosh...be sure to check out the new TrenD boutique..it's so cool!) before having a full evening! Maybe even check out your Resort's pool that morning before setting out. Last October we spent a lot of time at the pool because it was still very warm in Florida.
Your vacation will be here before you know it! I hope that you have a Magical time!