Hello Magda,
You have - and are once again - traveling many miles to get to the 'Greatest Place on Earth!'
Speaking from our family's experience - we LOVE
Fantasmic! Our children loved it too when they were the same age as your children. Fantasmic is a very 'big' fireworks and water show...played out in front of you over a shimmering lagoon. Mickey is the star of the show, and many of the beloved Disney characters that we all love are featured too (click the above link to find out the story and characters). You'll see the villains too...and the ending is spectacular!
This is a show unlike any other. It takes about a half an hour in total, but you will want to get there early (at least 60-90 minutes in advance) to ensure you get a seat (seating often fills up completely). To save time, our family often takes our seats and then we eat our dinner while waiting for the show. Also, there are Fantasmic 'Meal Packages' available where you can eat in a restaurant for lunch or dinner and then you will have reserved seating in a special area of the Fantasmic amphitheater. The benefit of this is that you will not have to get to the amphitheater as early - thus maximizing your time at Disney's Hollywood Studios.
Dress warmly as this is an outdoor experience. One little note of warning: you may not want to sit in the first several rows as you could get wet there - and it could be a little cool in December.
Fantasmic does not play every night. By July, the
park calendar should be available for December - check it out first so you can plan your visit around it.
Magda, we wouldn't miss this - it is a wonderful family experience. Enjoy!