A mother whose toddler was allegedly crushed by a car seat has been jailed for two years and nine months after being convicted of child cruelty and assaulting a witness.

Adrian Hoare, 24, from Gravesend in Kent, was found guilty of putting her three-year-old son, Alfie Lamb, into harm’s way by placing him into the footwell of an Audi convertible.

Her boyfriend, Stephen Waterson, from Croydon, has been accused of forcing back his car seat to squash Alfie in the footwell behind. The 25-year-old, the adopted son of the former government minister Nigel Waterson, denies the charges and faces a retrial for manslaughter in September.

The Old Bailey had heard how the child had collapsed on the journey back to Croydon from a shopping trip to Sutton, south London, in February last year. He died in hospital three days later.

Hoare, a hairdresser, was found guilty of child cruelty and assaulting Emilie Williams, who was in the car when Alfie was fatally injured. Hoare had also admitted plotting to pervert the course of justice after her son’s death.

In mitigation, her barrister told the court on Wednesday that Alfie had been a “happy child” and there were no issues of neglect before the events leading to his death.

Adrian Hoare. Photograph: Metropolitan Police/PA

Katy Thorne QC added: “Ms Hoare has been hugely affected by Alfie’s death, not least because she feels a huge amount of guilt over it.”

She received a sentence of two years for cruelty plus eight months for perverting the course of justice and one month for assault, all consecutive.

Mr Justice Kerr told Hoare: “This is a very sad case. You intended no harm to Alfie. You put him in danger by allowing him to travel in the footwell of the car. There was an element of deliberate disregard for Alfie’s welfare. I cannot ignore your own admission that you had allowed him to travel in the footwell many times.”

He said Hoare failed to tell paramedics and doctors what had happened and should have disclosed “anything that might have helped him”.

“I accept and bear in mind you lost Alfie, your only child. That is punishment already. I accept you are remorseful although it did not show very much because you were numb in your trial.”