There's a pretty good chance the game "Stop" would be the children's game known in the USA ,at least, as "Red light, green light". Red and green traffic lights would not be a phenomenon well known to Russian children by this time (1917-18), but the idea of sneaking up on a player without being seen , surely would be.
Btw, Inok Nikolai, do you have the date for this letter of GD Marie's?