The incident happened around lunch time yesterday, as the S Club 7 star visited shops in North London.
Onlookers claim her two daughters were left in the car for around 10 minutes.
One witness told The Mirror: “At first I didn’t realise they were in the car on their own or how young one of them was but when I did it was quite shocking.
“I suppose I think about the worst-case scenario but what if they took off the handbrake or managed to get out of the car?
“Or if someone managed to get in?”
Rachel previously revealed her trim figure is down to her busy parenting lifestyle, revealing:
“You don’t have time to eat when you’ve got kids!”
The singer, who celebrated her 6 year wedding anniversary with husband Alex Bourne last month, reunited with her bandmates for a UK arena tour in May.
Leaving a child unattended in the car is not illegal, and according to the official government website, the choice is up to the parent.
Advice reads: 'The law doesn’t say an age when you can leave a child on their own, but it’s an offence to leave a child alone if it places them at risk.
'Use your judgement on how mature your child is before you decide to leave them alone, eg at home or in a car.'
She has not commented on recent reports.