There shouldn’t be many, if any, holiday parties by December 21st. At the most, you may arrive on the last party day, so I wouldn’t factor those in so much.
This is a tough choice as I have had trip at both times of the year. Christmas week is extremely crowded and we typically find ourselves spending more time at the resort than at the parks, so be sure to select a resort you will enjoy due to theming and amenities, not just location. The weather is cooler and for some reason people seem less stressed even if the crowds are higher. The cooler temps probably help with that. Most annual passes will be blocked out, except for the most expensive one, for the bulk of your trip. The decorations are lovely and there is holiday music everywhere. The holiday parade that is run at
MVMCP runs daily at MK during Christmas week instead of the regular afternoon parade.
May into June is much hotter and more humid. Our wet season typically starts sometime in May, and there are usually a couple of hours each afternoon when storms will come through. When that happens the Skyliner and boat transportation will stop until it is safe to resume operations, so keep that in mind when selecting a resort. Crowds will be lower than Christmas, but you will have a bump in crowds if your trip includes Memorial Day Weekend. We typically aim for a resort break to coincide with any expected storms, but we don’t use the pools. Rooms should be cheaper than they are at Christmas, with some discounts in place if you qualify, and date based tickets should be cheaper as well. Most annual passes should be able to access the parks, based on their individual block out dates.
So for us it would be better temperatures vs less crowds, provided there weren’t any budget constraints. If there were budget issues, then May/June would be the better choice. You can have a great trip either time of year, so long as your expectations are reasonable. Only you know what will work best for your family