Kirby had some bitter disputes with Marvel and it's generally felt they treated him very poorly. He'd probably be happy to see his heirs get everything they could out of Marvel.
MacFarlane, Liefeld, Texiera, Bagley, etc, would have no grounds for legal action because the work they did for Marvel was after work for hire had been explicitly defined in law. That's not the case for Kirby.
As an aside, in the wake of the announcement that Paul Levitz will be stepping down as DC's head honcho, many creators have remarked that Levitz insituted a policy that was far more equitable towards creators than was previously the case and than has ever been the case at Marvel. Levitz made sure that creators signed the necessary documents that would give them the right to be compensated for the use, in any medium, of any character they created. Len Wein said that he resisted going to the trouble of signing the paperwork when he created Lucius Fox since the character was just "a guy in a suit", but Levitz insisted, and as a result Wein has been paid out quite nicely for the high profile use of the character in Batman Begins and The Dark Knight.
Levitz also reportedly came up with a settlement that the Siegel family was quite happy with at one stage, but the higher-ups at Time-Warner nixed it.