The Walton siblings from left, Alice Walton, Jim Walton and Wal-Mart Chairman Rob Walton, the children of the late Wal-Mart Stores Inc. founder Sam Walton, appear on stage during the company's annual shareholder meeting in Fayetteville, Arkansas, U.S., on Friday, June 3, 2011. Wal-Mart Stores Inc. renewed plans to buy back as much as $15 billion of its shares, potentially bolstering the Walton family's control of the world's largest retailer. Photographer: Beth Hall/Bloomberg via Getty Images

The heirs to the Walmart fortune, worth an estimated $140 billion, are one of the richest families in America, but a new report says they've given just 0.04 percent of their personal wealth to their charitable foundation.

Our Walmart, a group that fights for fair wages and other rights for Walmart workers, released a damning report on Tuesday of the Waltons’ giving tendencies. After analyzing 23 tax returns filed by the Walton Family Foundation, the group found that the company’s four heirs -- Rob, Jim, Alice and Christy Walton -- have given a total of $58.49 million to the family foundation, which comes to 0.04 percent of their net worth.

According to the report, the organization -- which has just under $2 billion in assets -- is 99 percent funded by charitable lead annuity trusts (CLATs), which offer the Waltons a sizable perk.

The money put into these trusts is designated for charity, but if the assets appreciate significantly, the money can be passed on -- tax-free -- to the heirs, according to Bloomberg.

A donor secures assets in the trusts for a set period of time and gives away a set amount each year. Whatever funds are leftover at the end of the period goes to the beneficiary, which is usually the donor’s heirs, without having to pay taxes, Bloomberg added.

The Waltons, Bloomberg reported, are "by far the biggest users" of such trusts in the United States.

By employing CLATS, the Waltons have saved an estimated $3 billion in estate taxes, a levy that generous moguls -- including Bill Gates and Warren Buffett -- support as a means to ensure that the wealthiest Americans pay their fair share to the government, Bloomberg noted.

But for donors like Gates and Buffett, giving a substantial amount of personal wealth is a critical aspect of the philanthropy game.

Back in 2010, Bill and Melinda Gates and Buffett launched the Giving Pledge, an initiative that encourages the world’s wealthiest people to pledge half their wealth to charity while they’re alive so that they have more control over how the money is spent. To date, the pledge has 127 signatories, including such heavyweights as Facebook COO Sheryl Sandberg, former New York City Mayor Michael Bloomberg and eBay founder Pierre Omidyar.

Still, despite the Giving Pledge’s earnest mission, some philanthropists have taken issue with the fact that it could potentially offer the same tax loopholes the Waltons have taken advantage of.

In 2010, Gates approached Robert W. Wilson -- a hedge fund manager who took his own life in February -- the philanthropist said he wasn’t interested in joining the Giving Pledge.

Wilson wrote in an email to Gates that he opposed the "loophole" that allows billionaires to simply set up family-controlled foundations that often become "bureaucracy-ridden sluggards."

Other experts have expressed similar sentiments.