A low-flying Dakota plane caused chaos in a showing ring at Lincolnshire County on 19 June.
The World War II plane was due to fly over the show at 12.15pm, but due to a delay went over later than planned.
Show staff say they were not informed of the change.
Bystanders said the plane was about 100ft over the ring where 19 first ridden mountain and moorlands had just started the class. It returned over the same ring 30sec later.
Four children were unseated.
“Ponies scattered, children were screaming, parents were running in. It was quite horrific,” said judge Helen Turner.
“There was no prior warning at all. We were given warnings about the Red Arrows display, but they flew over the car park.”
Mrs Turner said she gave the ponies time to settle and called them all back into the ring.
“We didn’t think it was fair for the children not to be able to start the class again,” she said.
The children were then stood down again due to parachutists coming into the main ring and letting off flares.
The steward for the class, Joanne Griffin, said: “If these attractions have to take place, lessons must be learnt regarding timetabling and communications. Ponies and planes do not mix.”
Jayne Southall of Lincolnshire Showground said: “In future, we will be asking all our aircraft partners to ensure they keep to time slots, in order to keep to our strict timetable.”
She was “very sorry” and said she will be making “tighter stipulations” on where planes can fly over the showground area “to prevent these incidents in the future”.
H&H columnist Stuart Hollings was judging at the show later in the day. His stewards told him to stand down the show hunter pony class for 10min for the Red Arrows display.
“On this occasion, at least we were aware,” he said.
For full Lincolnshire County report don’t miss this week’s issue of H&H out today (3 July).
This news story was first published in Horse & Hound magazine (3 July, 2014)