A tiny planet is not fantasy; the obstacle is scale. An asteroid only a few hundred meters across has surface gravity far below that of Earth, with escape velocity of just a few meters per second, so a strong jump could eject a child into space unless the environment is engineered with care.
The most credible fix is density, not magic. Make the body a solid iron or nickel rock, increase its mass while keeping radius small, and Newtonian gravity does the rest, raising surface acceleration and escape velocity without changing the storybook size. Add a slow rotational period so centrifugal acceleration stays well below local gravity, preventing equatorial regions from becoming zones where a sideways run could send the child on a ballistic trajectory.
Atmosphere is the hard bargain. A small object cannot easily retain gas against thermal escape and solar radiation pressure, so a stable breathable layer demands artificial containment: pressure domes, magnetic shielding, or a thin shell, all consistent with standard fluid dynamics and kinetic theory of gases. For the child, the real risk is not drifting away but crossing escape speed, which orbital mechanics defines precisely, so safety cables, recessed valleys, or even partial tunnels could keep each step bound to the rock while still preserving the image of a boy striding around his world in a single afternoon.
