Our 1st trip DD was about 18m and DS 3 yrs. We had a family room and it worked out very well. We were either going to drive or train as we didn't want to have to do the flying thing. Now that we've flown I don't think we'd do the train again, but for that age I would. Although it does take most of the day to get there, when you figure everything else into flying it's not like you really lose all that much time. It's great to be able to walk around easily.
When we went it was december, way down was a bit crowded, but coming back on Dec 23rd we basically had the train to ourselves. Food is better than plane food and the staff seems willing to do most anything for you