I'll concur with the Talkeetna and HQ recommendations. Talkeetna's a great little town for a stopover, with a neat little museum and eateries. Taking a Denali flight, especially with a glacier landing, is a tremendous amount of fun, though it is pricey. The first time we did it, MrsCP and I went on different flights as the little one was only 15 months old. (we took our 12/8-yrs olds with us last summer and they had a blast).

The Iditarod HQ isn't much to see, but is certainly worth a few minutes detour. For kids, the Reindeer Farm in Palmer was a big hit, and only a 15 minute detour total. The musk ox farm is also in Palmer.

working my way through the thread.....

I would agree that the Wildlife Conservation Center would take precedence of the Reindeer Farm and Iditarod HQ, if it's an either/or. But HQ and Reindeer Farm shouldn't take more than 90-120 minutes total, including the detour drive time. We spent several hours at the WCC. We also took a couple of hours to go up the Alyeska tram and do a bit of hiking.

If you're headed to Whittier for a cruise, stop and take the Portage Glacier cruise - a quick 1hr, and also look at the Portage Pass and Byron Glacier hikes along the way, if you're into hiking. Or even stop by on the way down/way up for the Portage cruise and Byron hike, where you don't need to worry about the tunnel schedule.

We had a great 1/2-day kayak with Kayak Adventures Worldwide, even if we did have to be rescued by boat when the winds swung around. Great people to deal with and we saw all kinds of wildlife - dozens of bald eagles, a seal that followed us for a while, etc. 5 might be a bit young without kayaking experience, you'd have to check their policies, though I've kayaked at home with my kids that young just sitting up front.