Heh, I did this with a party of adults only and we had a blast! The cast members didn't even blink an eye at the age of our Wilderness Explorers.
It took us two days to finish it, but we did it at a leisurely pace and stopped to enjoy the park, animals, and rides as we went (this was in the off season, might be harder to complete during busier times due to crowds). The booklet also makes a cheap, great souvenir, it looks just like a Wilderness Explorer guide. The cast members were also great...we noticed over the two days that the same cast members would get assigned to different stations throughout the week, so by the second day we had staff recognizing our group from the first day!
If I had children though, I'd probably wait until they were grade school, otherwise they would probably be too young to be interested in the activities.