Because it's grandma who found the art piece and not her grandkids and because grandma live in France [1], grandkids shall see approximately six millions EUR (divided by the number of grandkids) of the 20 millions EUR this would sell for.
Inheritance taxes above 1.8m in France are 45%. Grandma to kids: 55%. Kids to grandkids: another 55%.
I mean, three kids getting 2m each is nice but let's not kid ourselves here: it's the state that can thank grandma as over two generations they'll be taking 14 millions EUR out of 20 millions EUR.
And grandma is not free to do what she wants with the money: should she decide to give the money, the donation would be taxed at inheritance rate (45%: which I'm sure komrades find totally cromulent and probably even not enough).
(which shall greatly help a state-owned entity like the Louvre museum to acquire the masterpiece btw)
Uncle Sam (well, it's french cousin): "Thanks grandma!" indeed.
[1] I live in a country which has zero inheritance taxes and zero taxes when donating money and zero taxes on capital gains (if you held things like stocks/art pieces/collectible cars or whatever really besides real estate for more than six months). Income tax goes as high as 42% though but after you've paid your income tax, your money is yours and you basically do what the fuck you want with it.