DD will obviously need a nice nap before the party, and there's a couple of options. Perhaps you don't want to go to any park that day (save the price of admission), but could do Downtown Disney and the
Lego store and a fun lunch or the pool. Then a nap, and then get to the party ~ 4 PM. Don't wait until 6-7 PM to go --- there's longer lines to get in then. DD will have time to do more rides and attractions if you're there at 4 PM. It's like getting a bonus of 3 hours for the price of the party ticket.
Week nights tend to be less crowded than weekends, and it gets more crowded when it gets closer to Halloween. If you want a sit down meal in MK, book as soon as possible, and schedule it to end before the party starts, so you don't miss out on party events. Or, don't take the time ....... just take a break and get quick service.
If you're DO plan to go to a park that day, just allow time for a leisurely nap in the afternoon, and don't make major plans for very early the next AM (ie, Chef Mickey's at 8 AM). WDW is very overstimulating for many kids, and one of the toughest things is trying to have a good time with a tired and cranky child. Plan to be flexible.
If you can stay in any of the MK
DVC properties or MK resorts, they are obviously the most convenient for attending the party....... but be prepared for lines of people trying to get on the monorail and boats to get to the party. Another reason to go early. The Contemporary is less than 10 minute walk and you're not dependent on ANY transportation.
If you're staying at a resort that requires bus transportation, you may find that there's no wait at all to get back to the room .... especially if you don't try to leave with the crowd that will exit immediately after the first parade / fireworks.
Hope you have a great trip!