Given that the first 100k are not taxed and that there are three kids, only about 14 millions will be taxed at the 45% rate, not 20.
Note that we are talking about each kid making 6,67M€ (four centuries of minimum wage, three centuries of median wage).
If they earned their money through their labour instead of through their birth, they would actually be taxed more (IR + CSG + CRDS).
As for the grandkids, there are a bazillion ways to reduce the tax bill: life insurance is not inheritance-taxed, you can donate money to your kids before your death (about 300k€ per kid every 15 years before it’s taxed - if there are three kids, three grandkids per kid and everyone does a little tax planning you can give 9 grands kids * 300k€ * say give them money at 0, 15, 30, 45 and 60 yo that’s 13,5M€ originally from the grandma that won’t even count as inheritance, etc.).