Symbolism, Imagery, Allegory
It wouldn't be a spy story without a few gadgets
In Stormbreaker, these gadgets are hidden inside everyday objects that "a boy of [Alex's] age might carry" (6.54). Alex's yo-yo doubles as a grappling hook; his zit cream is actually a highly potent steel-melting compound; and his Game Boy is a multi-use computer that, in hindsight, isn't much more advanced than a modern iPhone.
In many ways, these gadgets become symbols of the contradiction between Alex's young age and grown-up spy adventures. Alex's age is met by skepticism by every authority figure he encounters, but he ends up proving them all wrong by the end. Maybe it's only because they underestimate him that this "bliddy schoolboy" (17.56) can slip his gadgets past Herod Sayle and his villainous cohorts. Just as his gizmos are disguised as children's toys, so, too, is Alex a full-fledged spy disguised as an ordinary child.