The woman, who cannot be named for legal reasons, appeared in Sydney's Downing Centre Local Court on Friday charged with knowingly giving false or misleading evidence to the commission.
The charge was unrelated to William, who was aged three when he went missing from a home at Kendall on the NSW mid-north coast in 2014.
The woman was accused of lying to the commission about whether she had ever struck another child in her care with a wooden spoon.
Magistrate Miranda Moody said she could not "discount the possibility she was mistaken and did not deliberately lie" to the commission, because she made such prompt admissions of hitting and kicking the child in other instances during the hearing.
The woman responded "yes" to questions put to her during the hearing including "did you ever hit (the child)?" and "did you ever kick (the child)?".
"Have you ever hit her with a wooden spoon?" Detective Sergeant Andrew Lonergan asked her.
"Never," she responded.
The magistrate said she "might have taken a different view had she denied ever hitting or kicking (the child)".
Audio recording from listening devices placed in the home recorded what police allege is the woman hitting a child with a wooden spoon.
The child can be heard threatening to call the police beforehand.
A woman is then heard telling the child to "stand up" three times.
"Where'd you put the wooden spoon?" the court heard a woman on the recording say.
The child then pleads, screams and cries, and is told to turn around and move her hands before smacking sounds are heard.
"She's still going on about it," the woman is heard telling her husband in a later phone call intercepted by police.
Earlier in the court hearing, Det Sgt Lonergan said he believed the foster mother knew where William Tyrrell is.
Her barrister, John Stratton said that was a false belief.
Police charged her for allegedly lying, in an attempt to pressure her, he said.
"You are hoping to break her spirit," Mr Stratton suggested.
"Our main objective is to find out where William Tyrrell is," Det Sgt Lonergan said.
The woman was emotional and teary when the magistrate delivered the decision.
She is due back in court next year charged with common assault.
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