When rich folks pass down their wealth to their children, they have to consider how to give to them. They usually have a schedule to distribute the money. For example, from 20 to 30, the kids can get $2,000/month, from 30 to 40, $3,000/month, etc. If the kids want to buy something outside the monthly payment, they can ask the trustees for the money, the trustees have the right to decide whether grant the requests or not. Then at a certain age, some trusts will let the kids gaining full control of the trust, and some family trusts remain as the trusts for the next generations, so on and so forth.
To protect money from the kids' divorces, they will have some clauses in the trust, the kids and their spouses can get money while they are married, etc.
The trust fund is managed by some trustees or by wealth management companies. The kids/beneficiaries usually don't have a say about the investment choises.