There is no statutory age to leave a child on their own but it an offence if it places the child at risk. The government have a website on the subject.
It’s not good practice to leave children alone in a car because something could have happened while you were away, you could have been an accident or the car could have been stolen. It has happened.
The way that the law would look at this is that at four years of age, whilst the children may not have been capable of starting the car with no keys in the ignition, they may have released the handbrake or something like that or opened the windows leaving them vulnerable.
There is very little chance that you will be prosecuted for this although you may get a visit from the police and/or social services if this lady does make that telephone call, but it would be to tell you not to do it again.
As the children were alone in, there is a slim possibility that you would be prosecuted for leaving someone in charge of the car who didn’t have a licence. However it is a slim possibility if the children were on the back seat.
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