The alleged offending between 2011-2013 was against six victims, most under the age of 14.
All six allegedly suffered physical abuse while two were allegedly sexually abused. The offences are alleged to have taken place in the CYF "family home" Kupa ran with his wife in Whangarei and two of the sexual offences at a home in Kaitaia.
Kupa had pleaded not guilty to the charges.
Opening the Crown case yesterday, Prosecutor David Stevens said CYF placed many children with Kupa, but he physically assaulted six of them - with the youngest, a boy of 5 - by kicking them, pulling their ears, hitting them, including with a foam bat, and getting other children to hit them.
He said that while having a takeaway meal Kupa made the "naughty kids" eat grass sandwiches and gave them a plate of stones rather than food.
"He told them not to tell anybody as 'what happens in the home stays in the home'," Mr Stevens said.
Mr Stevens said two of the girls, aged 14 and 15, were sexually abused, including rape, sodomy and other sexual violations.
He said Kupa told the girls he wanted to show them "how men and women do things".
The mother of one of the alleged sexual abuse victims broke down and sobbed in the back of the court, while Mr Stevens outlined the case and she had to be helped out of the courtroom.
He said the law allowed a parent or guardian to use reasonable physical force when disciplining children, but Kupa's abuse went well beyond that, while there can be no defence of consent in relation to the sex offences.
Mr Stevens said the children were placed in Kupa's care because they were considered vulnerable by CYF and that for whatever reason, their families could not care for them at that time.
The trial continues.