True, but the fact is that, if we assume the money left to their parent will remain until that parent's death; maybe even be invested/used for property and grow considerably in value (of course, it may all be spent on consumables in the parent's lifetime), then it will come to them as well as what they've already had directly.
I know this is MN, where anybody hoping to benefit someday from an inheritance is disgusting, evil and grabby (interestingly, when it involves siblings who are using potentially devious means to end up with far more than their fair share, they are not also accused of the same) - but in the normal course of things, I don't think it's at all unusual to assume that whatever is left from a parent's estate, when they do eventually sadly die, will be rolled along and become their children's money/assets - split equally between the children.
It's also very risky, from a fairness pov, leaving money to grandchildren as you can never be certain that all of your GC will have been born before you die, especially if you have sons.
I remember a thread from a long time ago (I may not be 100% on my memory of every detail). OP had two children whilst her sister was resolute that she would never marry or have children herself; so the grandparents left a very large chunk of their estate in trust to pay for the two named grandchildren's private education and then their university fees.
As it happened, after the grandparents died, the 'child-free' sister significantly changed her outlook on life and did go on to marry and have two children. She assumed that they would also be entitled to have private education paid for, as it had been put in trust 'for the grandchildren's education'; however not only was there not enough to see four children all the way through their education, but the trust also could not legally be used in any way to benefit the unnamed, at-the-time-unconceived grandchildren, so they couldn't even share it, so that they would, say, all go to state primary but then all go private from secondary.