It is alleged that she fed the animals a fast-acting sedative before the British Show Jumping Association event in St Lawrence.
The event had to be cancelled immediately for the safety of both horses and young riders and will be held later in the year.
Other parents and officials became suspicious when they saw the mother feeding mints to a number of horses.
It is alleged that the owners of two ponies saw something fall from one of their animals’ mouths and found an ACP (acetylpromazine) sedative tablet on the ground.
Blood tests have now been taken from both ponies, but the results have yet to return from the lab.