"A concerned motorist" told police he could see the children in the trailer, hiding under a tarpaulin.
He was travelling about 102km/h and said the car and trailer was pulling away from him.
Harris and Carberry then turned on to Pukepapa Rd, Marton, pulled over, and allowed the other three children to get into the trailer before continuing to Marton.
They pleaded guilty to the charges in Marton District Court yesterday before Judge Dugald Matheson.
Defence lawyer Scott Oliver asked that Carberry be dealt with differently to Harris as she was not the driver and was "less culpable".
"She didn't really know where they were going, she didn't know they were going to be going on the highway," he said.
A letter from Carberry's employer said the offending was "out of character" for her.
Carberry works as an RMA iwi appraisals officer, liaising with local government agencies.
Police prosecutor Sergeant Stephen Butler disagreed over her level of culpability.
"She was in just as much position to stop the driving as anybody," he said.
Anna Brosnahan represented Harris, who told police he didn't think it was unsafe when he left home but later realised it was.
Judge Matheson did not "see a need to differentiate" between the defendants.
"You are both responsible adults into whose care the lives of young children were placed.
"I have no doubt this has caused considerable embarrassment to you both," he said, adding it was "well-deserved".
Judge Matheson called the offending "foolhardy".
"I guess, this being a rural district, children have bounced along on the back of hay trailers since the start of the rural community."
He said this was "markedly different" to travelling on an open road at 100km/h.
"The risk, when one stands back to look at it in the cold light of day, is considerable."
He fined Harris and Carberry $750 and $130 court costs each, and disqualified each from driving for six months.