Is there a word or an idiom for people who only spend their families' money and fool around?

Is there a word or an idiom for rich people who spend only their families' money and do not bother to work, just fool around?

They are known as "trust-fund babies" or "trust-fund kids":

from Dictionary.com:

noun: a child of wealthy parents or other relatives who can rely on a trust fund rather than hard work for a living

Traditionally the most common (noun) terms for such a one would probably be a "prodigal" (as in the proverbial, Prodigal Son) or, alternately, a "profligate".

prodigal noun:

1. a person who spends money in a recklessly extravagant way.

• a person who leaves home and behaves recklessly, but later makes a repentant return.

noun: prodigal son

synonyms: wasteful, extravagant, spendthrift, profligate, improvident, imprudent; see Google prodigal

profligate noun:

1. a licentious, dissolute person. "he was an out-and-out profligate"

synonyms: libertine, debauchee, degenerate, dissolute, roué, rake, sybarite, voluptuary

see Google profligate

Indeed there is: "spendthrift" is one, and "wastrel" another. The latter is archaic but I really like it. Derived from "waste", I guess, but pronounced with a short A not a diphthong. You don't have to be stinking rich to be these, though. And there is another archaic word for spending an inheritance etc. quickly and reprehensibly: "to blew". That's not the past tense it looks like, because the past is "he blewed it all". National Lottery please note. No noun that I know of, though.

If your emphasis is not on blewing the money but just being a rich layabout, then, well, Bertie Wooster was a member of "the Drones Club". I think the British "Hooray Henry" and "Sloane Ranger" might fit. The American equivalent would I think be "trust fund babe". (Of which the masculine is what? I don't know,) Less demotic is "the idle rich".