Fresh back from a weeklong recess, Representative John W. Rose, Republican of Tennessee, was fired up and among the first to speak on the House floor on Monday, when he used his five minutes of floor time to castigate the criminal conviction of former President Donald J. Trump.

But the most memorable part of his earnest speech was not Mr. Rose’s words. Rather, a sideshow was unfolding behind him in the chamber entirely without his knowledge, courtesy of his young son, who put on a scene of silliness that quickly took off on social media, where dozens of viewers began circulating video of the shenanigans.

As Mr. Rose intoned the familiar lines of attack that rank-and-file Republicans have recited during television appearances and social media posts in the days since Mr. Trump was convicted on 34 felony counts, entering them into the Congressional Record, his son Guy, age 6, was putting on a performance of his own.

Seated directly behind his father and well within the frame of the C-SPAN cameras that record every moment the House is in session, the young Mr. Rose pantomimed and mugged, rolled his eyes and stuck his tongue out, and generally hammed it up in a display that is likely to be remembered longer than any of his father’s words.

It began normally enough.

“I rise today to address the terrible precedent set in our country four days ago, using the justice system to engage in a politically driven prosecution and now conviction of a major political party nominee running for president, especially on the charges brought against Donald Trump — should gravely concern every member of this body, as well as every American across our country,” Mr. Rose said as his son sat quietly behind his father.

But then young Guy appeared to notice the position of a television camera pointed in his direction, and began to have a little fun. It started with an earnest smile. Then a slight adjustment in his seat to guarantee full-frame exposure behind his father.