Brian, one classic CO hike is to get off the train at the Needleton stop. This gives you access to head steeply to the east, grunting your way several thousand feet up to the stunning Chicago Basin. Keep in mind that the train only runs once a day, and if you need to bail for any reason, you'd have to wait until the next train comes.
By the way, travel logistics from Durango to Aspen are goofy, probably a couple bus rides. You could always fly. But I think I'd not recommend the 4 Pass Loop (Aspen area)for an 8-year old--lots of ups and downs, where you gain/lose 2000' each day. Coming from a low altitude, I think this would be quite the strain on you both, but especially your son.
Another thought, maybe rent a place in Estes Park, and do several great day hikes in RMNP. Lots of people until you get higher up, but it's a national park for good reason. The Indian Peaks, as mentioned, offers scads of great hikes, some in-and-back, some scenic loops. All in all, I think the altitude will be a bit of a factor, if you're only going to be here for a week. Plan accordingly (including serious hydration a couple days before you get here), and acclimate wisely.
I'm not trying to scare you here, Brian. It's just that this thread drifted a bit, and folks were mentioning some hikes that maybe aren't quite appropriate for a low-altitude 8-year old. YMMV...