The number of children who are able to live off of their parents / grandparents money and don't work at all is so finite that it isn't really worth mentioning. Alongside that, they are a net gain for the economy.
Think about it - you have vast sums of wealth being dumped back into the economy through luxury purchases, and no impact to the labor force by them working. I may not agree with their decision to not work, but I really couldn't care less whether they do what I agree or not.
Also, a 100% estate tax would do one of two things: 1) create a disincentive for wealth accumulation as their is nothing to do with it afterwards thereby reducing their overall output 2) force those people into different investment vehicles where they can pass their wealth to their children as a "gift" prior to their passing.
That is really only import for those who are amassing large sums of wealth though. For the regular person - or even top 10% - all the 100% estate tax would mean is that dad's classic car that you restored together gets sold at auction instead being passed down, or the funds that could have gone to their grandchildren's college is now funneled into the government, or other similar act of smallish amount of funds being siphoned off by the federal government.