Ah, screw them. Seriously.
Unlike Siegel and Shuster, who were basically a couple of kids with no business savvy, Jack Kirby was a middle aged man with a twenty year career in comics when he went to work for hire at Marvel. He knew exactly what his contract terms were, having worked in the industry for his entire adult life at that point. Furthermore, he took no financial risk of his own in doing so (he got paid whether a book sold or bombed, unlike the publisher who took the actual risk) and he was more than willing to jump from company to company (Timely, Fawcett, DC, Harvey, Atlas, DC, Marvel) , depending on who offered him the best deal at the time. So you can't even factor in a "loyalty" argument. In fact, at one point in the 50s, he and Simon started their own company, which is further evidence that man was no babe in the woods who didn't realize what rights he was signing away.
Furthermore, when Joe Simon was in litigation with Marvel over Captain America in the late sixties, Kirby took Marvel's side.
Finally, even if Kirby deserved more remuneration for the characters, that doesn't mean his kids, who had nothing to do with his work, do.
