I never asked why ; I assumed it was a venerable tradition.
Visually, these young Pathfinders, or Russian Scouts, add a lot to the LIturgy by their presence.
They would guard Christ's sepulchre in shifts of I think 2 or 4 at a time. I have forgotten exactly.
Then each shift would be relieved by new Scouts.
The children add to the ceremony, whether standing guard or
changing the shift, for there is ritual and dignity emanating from these apparently well-disciplined Scout children.
No one giggles or looks inappropriately cheerful. They look strong [for their age] and determined to
perform their guard duties with aplomb.