Grennell was upset that he had to lie to his children.
"To tell my son what it actually meant — I can't. I had to tell him that was saying they love your school. I had to change the whole subject matter and lie to them a couple of days before Christmas — it is hurtful.
"The kids don't understand — they are too little to understand the emotion of the hate messages."
He said the children, who were fluent in te reo, had studied at the school for the past 18 months.
Mr Grennell said he always wanted them to appreciate the importance of Maori culture. "Maori culture has to be accepted throughout Invercargill and New Zealand."
He described targeting children with hate messages as crossing the line.
The Otago Daily Times was made aware of the same message sprayed on a roadside shed on State Highway 1 near Invercargill last week.
A police spokeswoman said yesterday they received a complaint about the incident.
"Police are taking this matter seriously, as we do all reports of hate speech or behaviour that causes concern."
She said police would be in and around the Newfield community providing patrols for reassurance.
Invercargill City Councillor Alex Crackett posted on social media about the incident.
She said it made her absolutely hoha (angry) and it was a "despicable act of blatant racism".
"I am ashamed as a community representative to see this occur in Waihopai, Invercargill.