Some random thoughts:

I have stayed at (and have recommended to friends) the Renaissance Flatirons, adjacent to the Flatirons Mall between Denver and Boulder. Do check prices before using points, as that location often has very good rates. There is a free shuttle to Flatirons Mall, which has a decent, sandy kids area.

I don't believe any of the Marriott properties in Boulder are within walking distance to CU or Pearl Street Mall (the pedestrian mall in Boulder, which has interesting kids areas). My guess is that there will be a lot of shuttling; hotels should have details. The Boulder Marriott is in a very congested area, and parking is likely to be very tight.

If you haven't gone through a lot of quick elevation changes with your child, be sensitive to potential problems in your planning. Boulder and Flatirons Mall areas are both around 5,000', while Trail Ridge (an easy day trip) tops out around 10,000', IIRC. Gaining and losing 5,000' in a few hours can be rough on ears, young or older.

Even if you are not into hiking (meaning backpacking), there are a many good short hikes which provide much better views that what you might see from the road.

There are some wineries and meaderies around the Front Range, in case those interest you (see the site below).

I find this site particularly useful: http://www.coloradolinks.net/

Considering the challenge of driving in the mountains, if you rent, you may prefer to rent a V6 rather than a 4 cylinder.

The Denver Art Museum is fasinating but has slanted walls which have a vertigo effect on some people. There have been some write-ups in the Denver Post about the issue.

2 nights in Estes Park (as mentioned above) should make for a few easy days in and out of Rocky Mountain National Park. At the time you are going (early May), Trail Ridge Road may not be completely open, depending on recent snowfalls. From Estes, you should have access to most of the east side of the park. Also, the road from Estes south to Nederland to Blackhawk to Golden is a very scenic highway, though going south, some of the best mountain views will be behind you at times.

As previously suggested, be prepared for a variety of weather, especially when heading up into the mountains.

Feel free to PM if you have questions you'd prefer not to post. There are several similar threads in this forum which probably contain additional useful suggestions.