He said the girl had been riding a 50cc child's bike, "smaller than the average farm bike. It's made for children, and there are many of them on the market. But all machinery is dangerous - you need to take care."
People needed to be aware that even a 50cc quad bike could be dangerous, Mr Glassey said.
"People should be mindful that they are a machine, just like a car, and can have accidents."
Mr Glassey said police were speaking to organisers about the sequence of events and were supporting the family.
"We can't say what's going on in their minds. It will be hard for them to deal with. We are not looking at this in terms of laying of charges at this stage, we just want to look after the family."
The death has been referred to the coroner and the serious crash unit is investigating.
The trail bike ride was held to commemorate Jayden Kilmister, a 15-year-old who died in 2009 after a gust of wind blew him off the back of a ute, along with furniture he was moving. All funds raised went to the Jayden Kilmister Memorial Trust, to aid local families in times of similar need, and organisations including the Westpac rescue helicopter and SPCA.
Federated Farmers Wairarapa president Jamie Falloon, who was at Sunday's event but did not witness the accident, said people were devastated.
"It was just a terrible accident. Kids ride horses and motorbikes. Kids do a lot of stuff and there's always risk involved, and we just hope that we never see the consequences. Wrapping kids in cotton wool is not the answer."