Charles Seeger, father of Pete Seeger, was himself an anti-war activist whose brother, Alan Seeger, was killed in WWI at the Battle of the Somme. A poet, Alan Seeger wrote I Have a Rendezvous with Death.
The picture, above, was also taken in late May of 1921 in Rock Creek Park in Washington, D.C., and shows Charles Seeger, his violin-playing wife Constance Edson Seeger, and their three children, including 2-year-old son Pete, sitting on his father's lap.
A May 22, 1921 article in The Washington Post about the family quoted Charles Seeger:
"We are trying to solve the problem of educating three boys, and at the same time lead a worth-while outdoor life."
Carry on Mr. Seeger; I am pretty sure you will not make a hash of this job!