It is already widely understood that playing personal audio equipment too loud can damage your hearing. This is usually mentioned as a safety warning in the product instructions. It is not clear what would be achieved by repeating the message on the external packaging.
These products are increasingly being supplied with software controls for limiting volume which parents can set for their children under a security code. Personal audio equipment intended solely for children is already limited to safe volume levels in line with harmonised European standard EN 71-1.