My grandparents had 5 children, who between them, gave them 9 grandchildren.
One of them had no children, one had 3, and the remaining 3 siblings had 2 each.
A similar conversation to the OP's was had with my grandparents around the time I was 18 (I'm the eldest grandchild), and in the end, my grandfather said he was sick of all the squabbling, and he was going to leave everything to the 4 youngest grandchildren, on the basis that he believed that the elder 5 of us would all be adults and earning our own money by the time he and my grandmother passed away.
As it turned out, he was right, my grandmother passed first, followed 2yrs later by my grandfather, at which time, I was almost 30 (and had 2 children of my own by then), the others being left out of the will were all mid-20s, whilst the 4 youngest were all still at school, and their inheritance was held in trust until they were 18.
I also remember something being said about leaving something for my 2 children (as they were clearly not adults), but my understanding is that my mother was given this, and it never made it to my kids.