Hi Rebecca!
With young children, I would consider taking advantage of the
Candlelight Processional dinner packages.
Dinner packages will save your family time in line. However, will they want to sit through a long meal earlier (or later) in the day, then sit through a 45 minute performance?
It's an amazing event in which the Christmas story is told by a celebrity narrator accompanied by a fabulous orchestra and choir.
If it's a busy night in Epcot, you may wait in line and still not get a seat for this popular event. In general, older kids will be more entertained than younger children. And whether they are able to sit through it depends entirely on their own unique personalities. Do they sit through other performances? Have they been to a band concert? When my son was 2, he was quite wiggly.
If you don't opt for the package, go on a weekday and try to get in to the earliest performance. It's usually the least crowded of the evening. Line up about an hour in advance. Ask a Cast Member if they think you'll make it into the theater as they are usually keeping an estimated count of guests.
I know that I haven't provided any definitive answers since all kids are different. I hope this info helps you make an informed decision.
Enjoy a magical vacation!