The television presenter and ChildLine campaigner was not present at her holiday home in the New Forest but had lent her house to friends in August last year.
Guildford Crown Court was told how the child was splashing about in the pool with Robert Gillings when she shouted out to her mother that he had touched her.
Mr Gillings, 63, of Addlestone, Surrey, and his wife were invited by the girl's parents to visit them at the house. Mr Gillings denies four charges of indecent assault, two allegedly at the country house.
Susan Tapping, for the prosecution, said the girl's parents asked her about the incident the following day and it became apparent that the defendant had sexually assaulted her. They reported it to social services after their holiday.
The jury watched a video filmed shortly after the allegations were made in which the girl talked about the pool incident and also claimed the defendant had touched her private parts on previous occasions.
Miss Tapping told the jury in her opening speech that in a police interview the defendant totally denied the allegations.
Ann Cotcher, for the defence, suggested to the girl she had made up the allegations and that the defendant had only touched her in the swimming pool while they were playing a game of tag.
The girl insisted that she had been assaulted and claimed that Mr Gillings also carried out a serious assault while they were in the changing room. She said the defendant had told her he would kill her mother if she said anything and he would deny it was true.
Speaking to Judge John Bull through a video link with the court, the girl claimed the defendant used to touch her on visits to his house. 'He would lift half my dress up and put his hand up. He would sometimes tickle and sometimes poke,' she said.
During cross-examination, Miss Cotcher pointed out differences between the girl's evidence and what she had told police.
Dr Susan Moore said she examined the girl and found two old healed tears in her hymen.
The trial continues today.Reuse content