This too goes beyond estate planning. I have siblings that have not had much success primarily by what I see as lack of trying and very bad decisions. My wife and I have worked VERY hard to be successful financially. The result is that my siblings’ kids (my nieces and nephews) receive things from my parents that my kids would never be given. Everything from college tuition to cars.
It is their money and they can do with it what they want, but when your kid asks why Grandma doesn't buy them a car, its hard to explain that they are better off earning the money to buy their own car. Which is what we expect them to do.
You have the wisdom of Solomon. You have some very lucky kids.