Animal-lover Anne King was visiting Wakehurst Place at Ardingly with a friend when the horror unfolded last week.
“It was awful,” she said. “I was absolutely beside myself.”
The drama started when Anne and her friend were standing by a lake in the botanic gardens - the country arm of Kew.
“We were particularly fascinated by seven fluffy ducklings which were barely a day old, with their mother nearby,” said Anne.
“I then noticed a boy and a girl aged about three years separating three of the ducklings from the group.
“The boy then started kicking gravel at one of the ducklings and frightening it.”
She added: “I was shocked and told the boy to stop kicking gravel at the duckling - whereupon he stamped on it twice.
“My friend imediately went to it. It was on its back and looked dead.
“I shouted out to the boy’s mother and expressed my disbelief and annoyance at the boy’s behaviour only for the mother to turn to me and accuse me of being a ‘stupid woman’.”
Anne, from St John’s Avenue, Burgess Hill, raced to a nearby coffee shop for help and two rangers helped her look for the family, “but they were long gone,” said Anne.
“The poor little duckling couldn’t get away,” said Anne, 58.
“I felt sick. It actually ruined my day. I couldn’t believe that this sort of thing could happen.
But my husband said later ‘You hopefully saved that duckling’s life’.”
And, in fact, rangers at the gardens later managed to locate the duckling - safe on the water with its mum and siblings.
A spokeswoman for Wakehurst said later that notices are now to be placed in the visitors’ centre asking people to be careful and vigilant around wildlife in the gardens.
“We were really upset to hear about what Anne witnessed,” she said.
“Mercifully, the duck survived but we are asking parents/family members to be particularly vigilant especially around areas near the mansion pond.”