2,265. Children and Yichud

Hilchos Issurei Biah 22:10

Seclusion is permitted with a girl under the age of three and a boy under the age of nine because the Sages only prohibited secluding oneself with a person with whom relations are efficacious.

Hilchos Issurei Biah 22:11

A hermaphrodite may not be secluded with women but if he is, he is not subject to corporal punishment because his status is in doubt. A man may be secluded with a hermaphrodite or a person of indeterminate gender.