I'm another who thinks that it should be divided between your children, and then it's up to them whether they pass any of their share to their own families.
You don't know whether there will be more grandchildren in the future, and it would be unfair to leave any unborn ones out of the will.
I would keep the split equal, unless you helped the older ones out when they were starting out - otherwise, you would be helping the youngest one disproportionately. I don't think that it is fair to look at the relative prosperity of individuals when deciding how to split a will. Unless your children all had very different starts in life, they have made their own way, and it doesn't seem fair to 'penalise' those who have worked hard. It would be different if family circumstances had changed when they were growing up and they started adult life in different ways; but if that's not the case, I would share it equally.
We made our wills recently, and went through all the thinking about grandchildren, and all the 'what if's that go along with that, and we decided to split whatever we leave behind between our children and let them decide what to do as regards passing it down to children of their own. We have explained this to them, and suggested that they might like to wait until they are sure that they have completed their families before doing so, but that in the end it will be up to them.