I'm a practical person and all for practical solutions, the most elegant solution is settlement out of court. Heck Disney/Marvel can spin it in such away that they could probably defray the costs as public relations.
I don't know how principled the Kirbys are but going for copyrights just seems like a waste of time. If they receive sufficient remuneration, they will effectively receive the financial support AND opportunities Jack worked so hard to give them in life. That has a nice sense of poetic justice to them.
If subsequently fumble the ball and squander the settlement, then that is their responsibility.
Now, the question is what would be a fair and feasible settlement?
Stan received a million bucks a year plus a hefty settlement recently so I figure Stan was paid at least 30 million bucks in the past 3 decades.
I would consider that fair for the Kirbys but I don't think Marvel/Disney would go for it.
Now if we look at Marvel's most significant, money-generating properties, I think it safe to say that almost everyone would agree that Jack made a significant contribution to:

The FF (including the supporting cast of important ancillary characters like; Silver Surfer, Black Panther, Dr. Doom, the Inhunmans, Galactus)
The X-Men (ditto; Magneto, the Sentinels) 
Thor (ditto; Odin, Baldur, Hela, Heimdall, Mangog, Loki)
Captain America (ditto; Red Skull, Batroc (okay, okay)
The Hulk

Of course strong arguments could be made about other properties but the above seem unassailable. The Avengers for instance was merely a collection of already established properties so I wouldn't describe Jack's contribution to the title as "significant".

I think a million bucks per  property is a fair so five millions dollars for the Kirby's seems to me to be a nice sum. Not large enough for Disney/Marvel to sweet it but big enough to be seen as a moral victory and more significantly, big enough to give the Kirby estate financial security now and - if they play their cards right - in the future.