I see what you are doing with your schedule, with Girdwood as an overnight on that long drive from Denali to Seward. I also understand why you picked those two endpoints--land mammals on the Denali NP bus trip and marine mammals on the Kenai Fjords NP cruise, both of which have animal sightings that are pretty much guaranteed.
So, first check out the Alaska TourSaver coupon book to cut costs. Then skip Talkeetna and definitely stay at Riley Creek Campground at Denali NP. That way you can leave the camper in its site all day and walk to the visitor's center to board the tour bus (ie., no car/toad needed at Denali).
Then drive to Portage Glacier and stay in the National Forest campground there. Or for a shorter drive, stay at the state park campground right off the freeway in Eagle River. Either way try to miss rush-hour traffic in Anchorage.
Then drive to Seward and stay in the city campground on the waterfront. That is the only place you can camp in town and stay in one place without a vehicle (ie., no car/toad needed). From there, you can walk to the SeaLife Aquarium and to the Kenai Fjords tour boat (although the marina will be a long walk with the kids).
And that's all you are going to have time to do in the short trip you have planned. No Talkeetna, no Denali SP, no Homer, and very little Anchorage--just not enough time on this trip and at the kids' ages for those stops. But next time! And, after being there for that short time, there will be a next time. Just ask anyone on here who has been to Alaska.