Adrian Hoare, 23 allegedly failed to prevent her boyfriend Stephen Waterson, 25, from squashing Alfie Lamb in the footwell of his Audi convertible with his seat in February last year.
Afterwards, Hoare and Waterson, the adopted son of former Tory Minister Nigel Waterson, lied to police about what happened.
Following an Old Bailey trial, a jury was unable to decide whether Waterson was guilty of Alfie's manslaughter, but found him guilty of intimidation of a witness in the case.
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Hoare was cleared of manslaughter, but found guilty of an alternative charge of child cruelty and common assault on another witness.
The defendants had previously pleaded guilty to conspiracy to pervert the course of justice.