The girls, aged 13 and 14, took the two-year-old child from the busy shop in Northumberland Street on 13 April.
She was found in a park three miles away in Gosforth an hour and 45 minutes later after the girls were tracked using CCTV.
Police said the girl had been with her mother when she was reported missing at 5pm, triggering a city-wide search.
The defendants, who cannot be named for legal reasons, admitted the abduction at North Shields youth court on Tuesday.
The girls also admitted two counts of theft for stealing dummies from Boots, and a baby bottle and milk from Sainsbury’s.
They had been charged with kidnap with the intention of committing a sexual offence but it was discounted after the prosecution accepted a plea to the alternative charge of kidnap.
At a previous hearing, it emerged that the girls had unsuccessfully tried to grab another small child, who was also black, at Primark on the same day.
Lee Poppett, prosecuting, told a previous youth court hearing at South East Northumberland Magistrates' Court that the the failed abduction happened two hours earlier in the same Primark store.
Her mother said one of the teenagers approached her and described her daughter, saying she was by the counter, but that she found the child in the opposite direction.
In April, a court hearing heard that a tablet computer was found containing searches for “rape”, “people getting raped”, “young people getting raped”, “poor little thing getting kidnapped and raped”.
Mr Poppett said the girls had been known to social services for “quite some time” and had gone missing from home on a number of occasions.
Chief Superintendent Laura Young, of Northumbria Police, thanked local police and businesses for their co-operation in the search.
“The two children charged with this offence clearly need a lot of help and support and we must now allow the justice system to do it's job and respect this process," she added.
“What is far more important is that the child is now back safely with her family and that is down to the fantastic work of police, our partners and the public here in Newcastle.”
Additional reporting by PA