I would spend maybe 1/2 day at Epcot... you could start the morning at Disney MGM, and then take the boat launch over to Epcot, which will let you out at the International Gateway, located between UK and France. Depending on height, your kids would like Test Track (40 inch min) and Mission Space (44 in min)-- you may need to get FP's for these and return later to ride. Then do the Kidcot masks in the World Showcase, ride Maelstrom in Norway, and grab something to eat in one of the wonderful restaurants. You can finish up your evening watching Illuminations!
Hope you have a great time