She said many children did not reach that height until 12 years old and only a small percentage of 7-year-olds would.
Ms Hefferen said last year checks of child restraints around local early childhood centres showed 41 per cent of children were not restrained properly.
"There's resistance from a large chunk of the community to use restraints at all in a variety of situations."
Plunket's national child safety adviser, Sue Campbell, said with five children killed or injured on New Zealand roads each week, the changes were a step in the right direction. "Without a booster seat, an adult seatbelt sits too high on a child," she said.
"The lap part of the belt rides up over their tummy and the sash part lies across the neck.
"So what we see is the seatbelt being placed behind the child's back or under their arm, the child sliding down in the seat and the seatbelt riding up against the squishy part of their tummy.
"In a crash, this can result in serious head and spinal cord injuries and horrific injuries to the abdomen including ruptured livers and spleens."
Silverstream mum Tessa Gillett said if the changes kept her children safe, she'd be happy.
"That's what it comes down to. If it's the best prevention then why not? What price do you put on a child's safety?"
Masterton police Senior Sergeant Carolyn Watson said most Wairarapa people used child restraints, but even one child not restrained properly was one too many.
"If you don't restrain your child and you're in an accident the risk of serious harm or death is huge," she said.
"I'm at a loss as to why anybody would put their child in that position."
An instant fine of $150 applies for having a child not properly restrained in a vehicle.
