WASHINGTON—When President Donald J. Trump welcomes President Xi Jinping of China to his palm-fringed Florida club for two days of meetings on Thursday, the studied informality of the gathering will bear the handiwork of two people: China’s ambassador to Washington and Trump’s son-in-law, Jared Kushner.

The Chinese ambassador, Cui Tiankai, has established a busy back channel to Kushner, according to several officials briefed on the relationship.

The two men agreed on the club, Mar-a-Lago, as the site for the meeting, and the ambassador even sent Kushner drafts of a joint statement that China and the US could issue afterward.

Kushner’s central role reflects the peculiar nature not only of this first meeting between Trump and Xi, but also of the broader relationship between the US and China in the early days of the Trump administration. It is at once highly personal and bluntly transactional—a strategy that carries significant risks, experts said, given the economic and security issues that divide the countries.

While Chinese officials have found Trump a bewildering figure with a penchant for inflammatory statements, they have come to at least one clear judgment: In Trump’s Washington, his son-in-law is the man to know.

Kushner first made his influence felt in early February when he and Cui orchestrated a fence-mending phone call between Trump and Xi. During that exchange, Trump pledged to abide by the four-decade-old “One-China” policy on Taiwan, despite his earlier suggestion that it was up for negotiation.

Now, Trump wants something in return: He plans to press Xi to intensify economic sanctions against North Korea to pressure the country to shut down its nuclear weapons and ballistic missile programs. He has also vowed to protest the chronic trade imbalance between the US and China, which he railed against during his presidential campaign.

China’s courtship of Kushner, which has coincided with the marginalization of the State Department in the Trump administration, reflects a Chinese comfort with dynastic links. Xi is himself a “princeling”: His father was Xi Zhongxun, a major figure in the Communist revolution who was later purged by Mao Zedong.

Not only is Kushner married to the president’s daughter, Ivanka, but he is also one of his most influential advisers—a 36-year-old with no previous government experience, but an exceptionally broad portfolio under his father-in-law.

“Since Kissinger, the Chinese have been infatuated with gaining and maintaining access to the White House,” said Evan S. Medeiros, a senior director for Asia in the Obama administration. “Having access to the president’s family and somebody they see as a princeling is even better.”

Former US officials and China experts warned that the Chinese had prepared more carefully for this visit than the White House, which is still debating how harshly to confront Beijing, and which has yet to fill many important posts in the State Department.

Several said that if Trump presented China with an ultimatum on North Korea, it could backfire.

“China will either decide to help us with North Korea, or they won’t,” Trump said in an interview with The Financial Times that was published on Sunday. “And if they do, that will be very good for China, and if they don’t, it won’t be good for anyone.”

The president said he had “great respect” for the Chinese leader, but he would warn him that “we cannot continue to trade if we are going to have an unfair deal like we have right now.”

Shortly after winning the election, Trump said he might use the “One-China” policy, under which the US recognizes a single Chinese government in Beijing and has severed its diplomatic ties with Taiwan, as a bargaining chip for greater Chinese cooperation on trade or North Korea.

Trump had thrown that policy into doubt after taking a congratulatory phone call from the president of Taiwan. That caused consternation in Beijing, and Xi refused to get on the phone with Trump until he reaffirmed the policy.

After the two leaders finally spoke, White House officials said in a statement that the men had “discussed numerous topics, and Trump agreed, at the request of Xi, to honor our One-China policy”.

Trump insisted on that wording, according to a person briefed on the process, because he wanted to make clear that he had made a concession to Xi.

Since that call, Cui has continued to cultivate the Kushner family. Later in February, he invited Ivanka Trump and the couple’s daughter, Arabella, to a reception at the Chinese Embassy to celebrate the Lunar New Year.

Inside the White House, the most visible sign of Kushner’s influence on China policy came in March at the beginning of a meeting of the National Security Council’s “principals committee” to discuss North Korea.

He was seated at the table in the Situation Room when Gen. Joseph F. Dunford Jr., chairman of the Joint Chiefs of Staff, walked in. Seeing no chairs open, Dunford headed for the backbenches, according to two people who were there. Kushner, they said, quickly offered his chair to Dunford and took a seat along the wall.

While administration officials confirm that Kushner is deeply involved in China relations, they insist that Secretary of State Rex W. Tillerson has taken the lead on policy and made many of the decisions on the choreography and agenda of the meeting at Mar-a-Lago.

In March Tillerson made his first trip to Beijing as secretary of state, during which he and Xi discussed the planning in a 30-minute meeting.

He was criticized afterward for repeating the phrases “mutual respect” and “win-win solutions,” which are drawn from the Chinese diplomatic lexicon and have been interpreted to assert a Chinese sphere of influence over the South China Sea and other disputed areas.

A senior US official said Tillerson applied his own meaning to those phrases—“win-win”, he said, was originally an American expression—and was not accepting China’s definition. He said the secretary had adopted a significantly tougher tone in private, particularly about China’s role in curbing North Korea’s provocations.

Kushner has passed on proposals he got from Cui to Tillerson, who in turn has circulated them among his staff in the State Department, officials said. But the department’s influence has been reduced as many positions remain unfilled, including that of assistant secretary for East Asian affairs. Though Tillerson has kept a low profile, officials said he was trying to develop his own relationship with Trump at regular lunches and dinners.

Kushner’s involvement in China policy prompted questions after reports that his company was negotiating with a politically connected Chinese firm to invest hundreds of millions of dollars in his family’s flagship property, 666 Fifth Avenue in Manhattan.

On Wednesday amid the glare of negative publicity, Kushner’s company ended negotiations with the firm, the Anbang Insurance Group.