Multisig (3/6 for example) fixes this. No need for trust funds or spreading around
parts of your seed (which could solve the no children part). It can be perfectly decentralized.
You probably want more than half needed for a multisig (where appropriate) or you run the risk of one half of the potential signers trying to race the other potential signers to take it all. Things often get nasty after people die when there's money involved.