I recently heard "land-ownership tax/renting land from the state instead of owning it" and "100% tax on death/abolish inheritance", I think they're interesting but not without problems.
One could argue that your kids did not earn that money either. As a reasonable middle ground, I think that we there could be a "good future for the children" exemption (at say $20m per child), while the rest is taxed at 100%. This prevents the natural process of concentration of all wealth in the hands of the few.
The process is called war. Properties are not valuable when tenants and landlords keep dying and the properties themselves might get destroyed.