He has pleaded guilty to one charge of indecency with a girl between 12 and 16, but has denied 12 charges of rape, eight of indecency with a girl under 12, three of unlawful sexual connection, and nine more indecency charges.
His trial began on Monday before Justice David Collins and a jury of five men and seven women.
The complainant in the 16-year-old video interview, who is now 21, told the Child Youth and Family worker how Bell told her to go with him, then pulled down her pants and "kissed" her "under there". She described it as "kind of cold".
Revisiting the alleged incident in another interview last year, the complainant said her only memory was a "freeze frame" of herself standing naked in the playhouse with a large amount of natural light coming in, while Bell, who was clothed, was nearby.
She remembered him being at her eye level as she stood there looking at a book with pictures of "bright, colourful flowers".
She said she was left with "quite a few blanks" in her memory.
Defence lawyer Fergus Steedman said Bell's position was that it never happened.
The complainant's father also took the stand yesterday and told Crown prosecutor Harry Mallalieu that Bell and his partner at the time had come to visit on January 3, 1999, and smoked cannabis oil with him and his own partner.
The girl wanted to ride her bike, and Bell offered to take the garage key and get the bike for her. The pair were gone about five minutes before Bell returned and the girl was spotted riding her bike, he said.
The complainant told him "about a week-and-a-half later" what had allegedly happened during that five minutes.
Mr Steedman pointed to the father's police statement from 1999, which said the girl had already been riding her bike outside and Bell went out for five minutes. There was no mention of the key or of getting the bike for her.
The witness suggested it might have slipped his mind, or the police officer who took the statement could have accidentally left the information out.
"Maybe because it wasn't true," Mr Steedman said. "Is that more like it?" The witness disagreed and said he might have said more that had been left out by the police officer.
Mr Steedman said the witness had last year read over his earlier statement and signed a further statement saying it was correct. The witness told Mr Steedman he had only "glanced" over the statement at that time.
The trial continues.