Using headphones for your child depends on factors like volume, for how long they are listening, and their ear canal size. When the child is young, his/her ear canals are small, so the sound level which is low for older kids can be high for your young child. The solution can be, let your child be old enough to decide and tell you whether it is too loud or too low for him/her. In all given conditions, do not let your child use headphones or earphones for longer duration at one stroke. It is harmful.