A three-year-old boy died after his mother's boyfriend pushed his car seat back in a ‘fit of childish temper’, jurors were told.
Stephen Waterson, 25, son of ex-minister Nigel Waterson, allegedly crushed Alfie Lamb either because he wanted more leg room or because he was annoyed, Duncan Atkinson QC said.
The prosecutor told jurors that Alfie's mother Adrian Hoare, 23, failed her son ‘fundamentally and fatally’ during the car journey to Croydon, south London, on February 1 last year.
The couple are on trial at the Old Bailey accused of Alfie's manslaughter.
In a closing speech, Mr Atkinson said Alfie was a ‘happy, active, smiley child’.
He warned jurors that they should guard against an emotional response to his death when they decide their verdicts.
He said: ‘No one is going to suggest either of these defendants wanted Alfie to die. In text messages between them they just wanted their 'little fatty' back, they just wanted him home.
‘Each of them may have called him a 'little shit' but neither of them set out to hurt him in the way that they did.
‘The question for you is whether Alfie died because without any thought for the consequences, Stephen Waterson moved his chair back, whether because he wanted more room or because Alfie was annoying him, putting Alfie at risk, lashing out at him with his car seat in a fit of childish temper.
‘The further question is whether the person Alfie was most entitled in the whole world to rely on when that happened, and had a duty to protect him from such harm, failed him fundamentally and fatally.
‘Failed him by putting him in such a dangerous place which is why children have car seats, which is why you would never think of putting a child in a footwell.’
The lawyer told jurors they must look at whether Hoare's failing to act was such a "fundamental dereliction of duty to her son" that she too could be held responsible for his death.
He said: ‘Alfie did not have enough room to breathe and that became the case after he was in the footwell and he was compressed in that way either for a short period followed by another period but in any view long enough to cause those irreversible injuries.’
He said neither of the defendants had explained the expert findings about Alfie's injuries ‘because they cannot admit what Stephen Waterson did and Adrian Hoare failed to stop’.
Hoare, who is originally from north Kent, denies manslaughter, child cruelty and common assault on Emilie Williams, who was in the car.
Waterson has pleaded not guilty to manslaughter and intimidation of the driver Marcus Lamb.
The couple and Williams have pleaded guilty to conspiring to pervert the course of justice by making false statements to police.