We did this last year when our kids were 7 and 4. We didn't need as much time in Liarsville as they gave us and the show was a bit cheesy, but the kids enjoyed it. The train ride was great, but the kids would have been bored had we taken the train back down. This excursion allowed them to stretch their legs, take a quick bus ride down and burn off some energy in Liarsville. It was a good combination for young kids.
The train ride is spectacular.