This was the longstanding WASP formula to get their kids to school Senior Prefect, Ivy, Wall Street and the Secret Service.
In a nutshell: a common practice (redshirting) well entrenched along the Boston and New York corridors of WASP power and high society social registry only now is trickling down to the plebs in the D.C. area private schools from the commotion in these Boards.
For centuries this practice was the common formula (and it still remains) to gain competitive advantage in exclusive NE prep schools (farm teams) ...leading to team captaincy, school leadership, lead in the annual school play and a move up to the Big Ivy Leagues where "Skull and Bone" societies and fancy eating clubs awaited before strutting onto the stages of power in America.
The underlying motives in D.C. are largely no different from yesteryear; despite the fact, in the long run, redshirts (particularly at the ages of 5,6, and 7) are no more smarter, more athletic or more charismatic than all the rest.