I've taken ambitious trips like this one, and I tend to enjoy it. Though I have not done so with kids!
I really doubt you can make it from Denali to Copper Center in 8 hours, but maybe you can if you do not stop much.
I would be tempted to take the 6 hour cruise based out of Whittier (or maybe eliminate it), then get on the ferry from Whittier to Valdez. The general idea would be Anchorage to Seward, then back to Whittier and ferry over to Valdez, then up to Copper Center and Paxton, over to Denali, then back to Anchorage. Or reverse the circle. That would result in less backtracking and thus slightly less hectic.
riftime