Alfie Lamb, three, was squashed when Stephen Waterson, 25, moved his seat back while his mother Adrian Hoare, 23, did nothing to stop it, it is claimed.
Giving evidence, Waterson's half-sister Samantha Dawson, from Blackpool, described Alfie as ‘the loveliest boy you could ever meet’.
Prosecutor Duncan Atkinson QC asked how the couple had treated Alfie when they visited Blackpool shortly before his death.
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Ms Dawson replied: ‘It varies on what mood they are in.’
She told the court when they were not in a good mood, they treated him ‘badly’.
She said: ‘They were always throwing him about and telling him off. (They would) pick him up and throw him on the sofa, both of them.
‘They would both literally just pick him up by his arms or slap him round the head. I saw both of them do it.’
The witness told the Old Bailey that Waterson had picked her up in a Vauxhall Astra and taken her to London a few months before Alfie died.
On that occasion, there were three adult passengers, including Hoare, with Alfie and another child in the footwell, the court heard.
Ms Dawson told Waterson the children should be in car seats but he responded, ‘Ah, they will be ok’, the court was told.
She told jurors Alfie could not get comfortable because he did not have much room but Hoare said he was alright and they would be there soon.
The witness added: ‘I heard a connection to his head, a slap land to his head from Adrian.
‘He was crying and screaming. She said 'shut up crying or I will give you something to cry for'.’
Ms Dawson was giving evidence via video link from HMP Styal, in Cheshire, where she is serving 30 weeks for a benefit-related offence.
Hoare and Waterson, from Croydon, south London, have denied Alfie's manslaughter.