Green’s Dictionary of Slang

kindergarten n.

1. (US Und.) a reform school.

1950

Goldin et al. DAUL 117/1: Kindergarten. A reform school; any juvenile place of detention or institution for adolescents.

In compounds

kindergarten cop (n.) [US movie Kindergarten Cop (1990)]

(N.Z. prison) a paedophile.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 100/1: kindergarten cop n. a paedophile.