I can't believe folks are saying to skip Denali Nat'l Park!! It's a treasure.
On our last trip we stayed 10 nights in the park. We saw 'the mountain' 7 of 10 days in full glory. We took the school bus shuttle tours and saw every one of the big animals who live in the park and tons of smaller ones. The bus drivers are very good at spotting them as they know where they might be from all the previous trips they've driven. They will make many stops for you to take photos.
We would recommend that you stay IN the park itself not at one of the parking-lot type of RV parks on the outside. You'll see animals in the campgrounds.
Seward is very easy to walk around. Try to get a site at the city campground right on the waterfront. You'll see playful otters in the water, seals and huge cruise ships (especially pretty at night if you're awake). The kids would love the aquarium in town.