I agree that the son with three kids is a CF - I'm only surprised that he didn't suggest splitting it 60/40, based on the grandchildren - even though the money isn't being left directly to the GC.
Maybe he wanted to do that, but didn't dare to push his CFery any further - or he did try that and the parent objected.
It's always a risk leaving to (or based on) your GC, as nobody (obviously unless they don't have any of their own children) can ever know that they won't have any more GC after they make their will or die - especially if they have sons. When you base it on your own children, you can be certain that, once you die (or a maximum of nine months after your death), you will definitely not become the parent of any more children.
All this is moot, though, as it's being left to the father of the GC and not to them. He could spend the lot himself, or it could all be swallowed up by care costs, and there be nothing left to them anyway.
We knew a family where the surviving elderly spouse left everything equally split nine ways to each person as follows:
Son 1 / Wife / DC1 / DC2
Son 2 / Wife / DC1 / DC2
Son 3 (bachelor, no children)
None of the adults better or worse off than the rest.
Son 3, who is a very generous, selfless soul, thought this exceedingly fair, even though, if split 'traditionally', he would have had one third to himself instead of one ninth.
Son 2 argued that it should be split three ways between the sons only; not because he thought that would be fairer for Son 3, but because he personally would have preferred to have one third all for himself rather than his own family have four ninths - getting on for half of it - between them.