A sphere of flame 20 feet in diameter with a humanoid baby form at its center
A Sun's Child initially appears as a sphere of flame, essentially a miniature sun 20 feet in diameter. This however is not its true form, which is that of a humanoid baby, with a glowing white hot surface, located in the center of the 'sun'.
A sun's child attacks by charging, bringing foes within its flames. These flames will burn any flammable objects and can melt most mundane metals (iron, etc.). Magically-enchanted items will be spared however.
To hit the sun's child the attacker must have a reach or range of at least 10 feet or else will fall within the sphere of flames and become engulfed in it.
Basic Fantasy Field Guide Omnibus