We're going to WDW first week of December to avoid crowds, but still get the holiday experience. I read that there's a Holiday Party where Magic Kingdom stays open later with parades and shows and rides being open. However, it costs like $90/person.

-Our kids are 4 and 2, so would it even be worth it for them at that age, and that late at night?
-The Holiday party ticket gets you in from 4 PM-12 PM, but since we already have tickets (a 4-day park military park hopper we have from earlier this year where we only used two of the days) is there a way to upgrade to the Holiday party without paying the full $90?
-Is it worth it, and are there any tips or discounts? Any difference between Disneyland and Disneyworld, in terms of value?