mere child; small child
Idiomatic expression emphasizing that even a young child can understand or do something, often used in contexts of simplicity or obviousness.
Even a mere child knows that.
子供 is the general word for 'child', while 三尺の童子 is a literary set phrase used to emphasize that something is so simple a child could understand it.
三尺 (three shaku, about 90 cm) refers to the small stature of a young child; 童子 means 'child'. The phrase is a classical Chinese-derived expression.