EMH (extra magic hour) can be a great way to see several more popular attractions at a relatively uncrowded time. EMH is definitely most beneficial in MK. MK has several attractions open during this time and MK will likely most feel the effects of the pre-Christmas crowd.
If your children tend to be very early risers, then try to make one or two EMH, especially at MK. Stay at the EMH park until late morning or whenever it begins to feel too crowded. Then, depending on the ages and nap requirements of your children, go back to your resort to rest or hop to another park. If you do go back to rest, then return to a different park (non-EMH) for the evening.
If you think it will be too hard to get your kids up that early, then don't stress going to EMH. Just be sure to visit only non-EMH parks that day. You still should try to arrive by opening time as the crowds will build through the day.