Historic train station first opened in 1881. Catch the Light Rail, city bus, 16th Street Mall Bus or Amtrack.
While visiting my parents in Denver we took my 3 and 1 year old sons for their first 'train' ride on Light Rail. We started in Denver and went out to the suburbs. The people at the station were friendly and the boys enjoyed looking at everything. The trains do not take credit cards! The trains were not full. It is an easy and fun way to get around with kids.
Denver Union Station, while being transformed to serve as the hub of lightrail, train, and bus transportation for the Downtown Denver area servicing the greater metro area, is a true piece of history worth enjoying even if just for a few minutes. My kids were in awe as we entered Union Station, the very large and grand wooden benches in the main hall towered over them. The kids had a blast sitting on the benches while they "waited" for the train. We were really waiting for grandpa to finish up in the bathroom before we headed to the Light Rail, but an imagination opportunity is always welcome. And yes, they have public bathrooms (a great hint if riding the light rail following a baseball game at Coors Field).
Union Station also has a collection of model trains in the basement. These were very interesting to look at, but a bit difficult for the kids to see without being picked up. There are several organizations that also have offices and historic displays in the basement, however, they were locked while we were there. I believe they are only open on Friday nights, but couldn't confirm this.
I wouldn't go out of your way to tour Union Station, but if you are travelling past or through (Light Rail and the buses will take you in the tunnels underneath the station that used to service the various tracks in the rail hay day) make a point to go in for a quick look and a jump back in time.