Sounds like an easy choice to me, nccb. If this is your family's first train trip, why not start it on a high note? Thirty minutes out of Denver, you're entering a remarkably scenic world of tunnels, pine trees and broad prairie vistas. After that comes the high Rockies, an awesome granite canyon, a sublime redrock canyon, several deserts, and finally the epic, scenic crossing of the Sierras. Thirty minutes out of Chicago, or even three hours, you're seeing acres of corn. The kids are wondering why they came. Point is, the Zephyr ride west of Denver is entertainment; east to Chicago, it's just transportation.
Also, two nights on a train, for the same price in points for one zone of travel, is a far better deal than only one. By the second night, everybody should be more relaxed and comfortable with the accommodations.
Like you, I'm planning to take a one-zone trip in late March, on points. It's our family's first overnight train trip together, so I can relate to your story. We'll go all the way to Vancouver from Denver. I could never afford the ticket price, with sleepers, but the points make it easy. Amtrak redemptions must be one of the most profitable ways to use credit card points.
To The Traveler-- The rail line to DIA is under construction right now, as is a large hotel at the airport. Rail service is set to begin in 2016, though I can't imagine how it would take so long. Bridges are under construction, and in the open land approaching the airport, catenary is being constructed. As a local, it's interesting to see how DIA's remoteness and lack of rail connection influences and deters tourists like nccb.