Green’s Dictionary of Slang

Justice Child n.

In phrases

do Justice Child (v.) [Sir Francis Child (1642–1715), banker and lord mayor of London]

(UK Und.) to act as an informer.

c.1698		B.E. Dict. Canting Crew n.p.: I’ll do Justice Child, c. I will Peach or rather Impeach or Discover the whole Gang, and so save my own Bacon.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].