The involvement of Trump family members has been a departure from the approach of non-familial Trump advisers, who have refused to comply with the committee.
Ahead of the event, legal experts called for Trump to be prosecuted to disqualify him from running again in 2024.
The ex-president's niece said her 40-year-old cousin could have "damning" information about the Capitol riot.
Attorney General Letitia James' office issued subpoenas seeking testimony as part of a civil investigation into the Trump family's business practices.
"We will be asking the court to compel Donald Trump, Donald Trump Jr., and Ivanka Trump to testify with our office under oath," James said after the Trumps filed a motion to quash upcoming interviews.
The president and his relatives have long been his most ardent defenders, and have continued to be so following his time in the White House as they seek to shape perceptions of his legacy.
One former official remarked that the ceremony "felt like it was a funeral," while another said "the country died," according to ex-deputy press secretary Gidley.
Ivanka Trump, the president's daughter and advisor, wrote on social media: "The last four years have been an incredible journey ... I leave D.C. in awe of the compassion, grace and grit that exists across the nation."
The president and his allies will be addressing the convention, as they aim to boost his reelection hopes.
Long before he became a president notorious for mocking people, Donald Trump's "name-calling and teasing kids too young to fight back" helped land him at the New York Military Academy, according to a niece's tell-all book due out next week.
A second lawsuit has been filed in an attempt to block the publication of Mary Trump's tell-all book about President Donald Trump, which is slated to be released July 28.
The president's eldest son also said he believed his father's critics would say the president had dementia if he mixed up states.
The president's son said The Trump Organization, which he runs in his father's absence, has lost out on international deals.
Fred Trump Jr. died in 1981 after battling alcohol addiction. He was under pressure to abandon dreams of being a pilot to join the Trump Organization.
John McCain's daughter says her "beef" with the Trump family is "forever."
"The U.S. is experiencing threats to its system of checks and balances as well as an erosion of ethical norms at the highest levels of power," according to the anti-corruption watchdog Transparency International.
Jeff Sessions's temporary replacement as attorney general has criticized the Mueller investigation for widening the scope of its inquiries.
Dr. Bandy Lee also said staff from inside the White House contacted her to say President Trump was "unraveling."
"I've been shooting with him, and I mean, he's the best I've ever shot with, for sure," Guilfoyle said of Trump Jr.
"All of them are. It goes with the last name," former GOP strategist Steve Schmidt, who recently quit the Republican Party, said of Melania Trump and the first family.
"She can't control the narrative anymore because she's so inauthentic," the author of "Born Trump" writes.
The president told reporters he used to discuss nukes with his uncle "all the time."
A preliminary investigation suggested that the substance was not hazardous.
Melania Trump was the only Trump family member with a higher favorable than unfavorable rating.