Coolgran, I'm sure your friend means well. But I take it you've already made this decision with DH and you say all the DC are fine with it. I wouldn't change a thing.
Lyndyn, I trust you realize that your SC are under no obligation to leave their inheritance to your GC by other children. And that once they inherit, the money is theirs to do with as they choose.
I can only speculate as to why one SC made a point of telling you where their money would go. It might, in fact, be to let you know it won't be going to your GC/their nieces and/or nephews, as you and DH might have hoped/expected. That SC may not want the GC or their parents to have false expectations or be hurt/shocked/disappointed, down the road.
If you want your GC to inherit a little more from you than just what they will get through their parents, you might set things up so they inherit directly from you. But that would, of course, mean having a little less to divide between their parents and your SC.