Are Children 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.
Sources:
אג”מ או”ח ח”ד סימן כא אות ד.
וראה בארוכה בשאלה 558:
Are my children allowed to have a swimming lesson on ג’ מנחם אב?
ובשאלה 3797:
#15545