During our sister's trip to Denali we decided to do the ATV Adventure tour. It was late August/early September, so it was cool outside and had been raining all day. We love being outdoors, but didn't have enough time to do a hiking trip or similar. The booking process was easy, and we were able to get a two-seater ATV (I drove).
The entire trip was very fun, starting with our short bus ride out to the ATV site. We were informed on how to stay safe, use the ATVs, helmet fittings, and a test drive through a sample road to make sure we felt secure driving the ATV.
We toured three different sites, with plenty of time to stop and take pictures as needed. We had three guides with us, an approximate party of 12 ATVs. All the guides took turns telling us about where we were going, stopping and general items of interest. All three were very friendly, informative, and just seemed to get along with each other and all the guests. We all felt safe, especially considering a young child of about 5-6 was on the tour. They were able to supply a booster seat and she seemed to have a great time.
Overall, I would highly recommend ATV adventures as an activity while in Denali. It is definitely suitable for young children and those young at heart.