A literal translation won't work. A couple of idiomatic expressions with a similar meaning, assuming you mean that one:
un fils à papa
un gosse de riche
Both are derogatory. In France, "trusts funds" are not really known or used, so translating it to fiducie would lead to an unintelligible expression. The fils à papa and gosses de riches get their money from their family regardless of the method used.
Here is another trust fund kid definition from quora that more or less match my suggestions:
A trust fund kid would be someone who was the beneficiary of such a trust. You usually hear it used in a derogatory way to imply that the trust fund kid has probably never had to worry about money their entire life, because they have sufficient income from their trust to live a very comfortable or even extravagant lifestyle.
Expressio translates fils à papa to AmE Richie Rich defined by the Urban Dictionary as:
An adolescent/teen/young adult who grows up in a well-to-do family often perceived as spoiled and often tries to get by on family wealth lacking any discernable talent or skill of his/her own.