There's a big thing missing from this picture: If you have $ 1 billion, then you will eventually have $ 10 billion (40 yrs of 6 % returns), and your kid could easily end their life with $ 100 billion (add 40 more yrs). Is that ok?
The parent paid their income taxes on the money already. I believe in his death he owes capital gains on his stock holdings. So all the money that’s passed down has been taxed fairly already.