In many jurisdictions, children under the age of 16 are not allowed to engage in regular full-time jobs; they may however be able work certain specific types of limited positions during certain hours like babysitting or working on a family farm.
Additionally, most places allow for limited-hours part-time employment after specific ages - but usually not before the age of 14 with many places setting 16 as the legal employment age.
For all minors there are additional restrictions around hazardous work, night-time hours and daily/weekly hour quotas to ensure the work doesn't negatively impact your child's education, health, social and emotional development while keeping in mind their specific developmental needs.
Given these many variables it's always best to double-check before hiring any minor employees or allowing an individual child to gain employment as there can be severe legal penalties both civil and criminal depending on your locale.