Katie Holmes reportedly wants to establish "financial security" for Suri.
The actress and her ex-husband Tom Cruise recently settled the terms of their divorce.
Tom is said to be paying Katie $400,000 a year in child support for their six-year-old daughter Suri.
Katie is thought to be putting a similar amount of money away annually for Suri to retrieve when she grows up.
"Katie wanted the trust fund put in place for Suri so that their daughter will be able to tap into after she turns 18-years-old," a source told Radar Online.
"Katie has absolutely no access to the money and didn't want to. She just wanted to ensure that Suri would have financial security as she becomes an adult. It's a sizable trust fund, but Suri won't be able to fully legally claim it until she is in her 30s. Tom had no problem with the request and planned on doing it for Suri anyway."
As part of the annulment agreement, Tom must also fork out for other costs relating to Suri including, education, medical, insurance and extracurricular activities.
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