Are Children (ages 3-5) allowed to listen to music during the 3 weeks?
Children should not listen to music in the 3 weeks.
If they did not reach the age of Chinuch (2-5 years old), and there is a need (to distract and calm them) one may be lenient.
In any case, when permissible, adults should not pay attention to it.
אג”מ או”ח ח”ד סימן כא אות ד.
וראה בארוכה בשאלה 558: