Both a charmingly old-fashioned period piece with a game cast (including two child actors - Christopher Beeny and Anneke Wills - who later had adult careers) beautifully shot on location by Denny Densham in the English countryside, and an extraordinary relic of the period between 'Seven Days to Noon' (and released the year the British government started work on building it's own H-Bomb) and the formation of the CND, with Atom Bombs treated as exciting toys rather than harbingers of the end of the world.
Anybody who thinks the British public were unaware of the Atomic Bomb will find themselves partially disabused of that idea by this extraordinary Group Three production directed by a woman in which the kids talk about little else without having the first notion of how destructive Atom Bombs actually were, or how dangerous radiation really was; and used a piece of rock from Krakatoa to build their bomb the way Dan O'Herlihy employed slivers from Stonehenge thirty years later to slaughter the youth of America in 'Season of the Witch'.