Is it considered a deprivation of rights and an injustice to a child if he is not paid for cleaning a car without prior request, under the pretext of forbidding him from imposing his services on people?

In summary: If it is customary in the questioner's country that the car owner's failure to signal the child to stop cleaning means permission for him to work for a customary wage, then the child is entitled to the wage. The questioner's refusal to pay it is considered an injustice and an usurpation of rights. He must repent to Allah and return the right to its owner. If that is not possible, he must donate the amount on the child's behalf.