These kids are playing the ‘diddley bow’ (various spellings), made by stretching a wire across a board and plucking it rhythmically while running a bottle or piece of metal up and down for the melody. The wire could also be stretched across a wooden wall on a porch (American for verandah). One of the forerunner instruments of blues, diddley bows are said to still be used in African American blues in the South.
These diddley bows were played in 1943 at the Fort Valley Music Festival, Georgia. (Photo from The Peachite, Vol 11, No 2, March 1944, p. 8 at the Library of Congress.
You can hear the diddley bow, and various modern adaptations, on Youtube and the Wikipedia entry has some useful links and references.