Just out of curiosity, why does any average person care if the 900 million loses a little of its value? Will that make your new yacht unaffordable? 
Following phrases of the sort “If you think X is going to happen, then do Y.” is a guaranteed simple way to get extremely wealthy which is why everyone in the world is mega rich now.
Question:
Do I have this right? If I suddenly had an after-tax windfall of $400 million, I could probably invest it in some way that yields, at very low risk, $20 million a year in interest (i.e. 5%) without losing any of the original principal?
Then each of my kids could have $100 million when I die, yielding $5 million a year in interest?
(Except inflation isn’t taken into account here though…)
Inflation risk is a thing to prepare for, yes.
Also, better to assume a 4% return in your equation to be safe.
Not at very low risk, but if you accept historical risks of the stock market, and put some money into bonds, and reallocate to take advantage of down periods in the stock and/or interest markets, there is no reason you couldn’t have $20 million a year in gains/dividends and interest and still leave $100 million to each of your children. Inflation is taken care of because you’ll actually make about 8% in the stock market, so if you spend 4% of it, you should be able to keep up your standard of living.
Given the large numbers and the impact of compounding, if you ONLY lived off $5 million a year, you could leave your children with even more money.