She said the sexual abuse was streamed live via a webcam to a woman in San Francisco who thought the child was aged about five.
She said the child was "most probably drugged at the time" and unaware of what was happening to her.
Miss Hope told the jury that the evidence in the case was "extremely distasteful and downright revolting", the Daily Mail reported.
Miss Hope said the woman in California is "by her own admission a paedophile", adding that the first of around five webcamshows happened in late 2012 or early 2013.
The prosecutor said: "It was a webcam connection. She was able to see both Craig and Sarah naked in bed."
She says a child was lying face down on the bed wearing a nightie.
"She describes seeing Craig and Sarah engage in sexual intercourse on the bed with the child next to them asleep."
Miss Hope said while Sarah Gotham was engaged in having sex with Craig Forbes she rolled over and used her left hand to lift up the child's nightie and "started to caress the child's bottom while having sex".
When the child stirred the contact stopped and so did the streaming because the woman in the USA did not want to see the "little girl freaking out".
The jury heard on another occasion Gotham was "home alone" when she performed sex acts on the child.
The court heard the room was lit well enough for her to see "who it was doing it".
The court heard that the sexual abuse happened on occasions when 36 year old Forbes was with Gotham.
The jury heard that the FBI was investigating child exploitation and abuse and came across Skype names including one called Bad Girl Next Door.
That led them to the door of the woman in California, who admitted that she was in contact with other paedophiles.
Miss Hope said that the woman said she had engaged with a couple in the UK called Sarah and Craig.
She claimed to agents that they put on shows for her to watch of them sexually abusing a girl.
The FBI contacted Devon and Cornwall police who then investigated Gotham and Forbes who lived in Plymouth.
The woman in California said they were "an attractive couple" and Gotham was "chatty and used a display name 'Sarah Girls Only'."
They used Skype ad video calls to chat because they wanted to ensure they were talking to "real paedos" and not being entrapped by police.
The jury heard they were chat logs giving "very graphic descriptions of sexual abuse of children they found sexually arousing".
Gotham, 34 today, denies six charges of sexual assault on a child under 13 between December 2012 and March 2013.
The trial continues.
