The night featured attacks against Muslim Dems and tributes to Charlie Kirk.
Capitol Police said they temporarily locked down the headquarters of the Republican National Convention after a suspicious package with two vials of blood was found.
"While Barron is honored to have been chosen as a delegate by the Florida Republican Party, he regretfully declines due to prior commitments," the statement from the former first lady's office said.
Barron joins several other family members - Donald Trump Jr., Eric Trump and Tiffany Trump - who were also selected.
Republicans on Friday chose Milwaukee in swing state Wisconsin to host the 2024 national convention, beating out Nashville in deep-red Tennessee.
The identity of who placed pipe bombs at the DNC and Republican National Committee headquarters in Washington remains a mystery.