The jury at Southwark Crown Court heard how Rolf Harris was a "dirty old man" who assaulted a child at a community centre near Portsmouth at one point between 1968 and 1970.
The woman, speaking behind a screen, said initially she was confused and thought there must have been some kind of mistake:
It was very quick. In fact so quick that I thought, 'what's happened?'. The first time I thought, no, not deliberate. I thought, 'what's going on?', because there were a lot of people around and I didn't process what had happened.
She went on to describe how he repeated the assault, "more aggressively", and broke down as she said she "wasn't the same child" afterwards.
She said Harris had performed Two Little Boys at the event. Harris denies all 12 counts of indecent assault on four alleged victims.
It didn't matter if it was going to hurt me or not. It felt very agressive and I knew that it wasn't an accident.
More top news
He said NFL stars who refuse to stand for the national anthem in silent protest over human rights should be fired.
The children - who are two of three triplets - were found not breathing at a property in Bridgend, Wales.
Jeremy Corbyn hopes his efforts to give Labour members more power over the party will help him sweep into Downing Street.