Ask your child’s Audiologist if they have any suggestions. Hearing in the car is a very challenging listening environment.
Speech-to-noise signals are less than optimal for speech understanding. First, try to minimize background noise, e.g. keep windows closed, turn the radio down or off and avoid competing conversations simultaneously.
If your child has hearing aids, the audiologist may be able to provide a special program for communicating in the car or may be able to offer you an accessory such as a remote microphone or portable loop system.
FM systems and other Assistive Listening Devices (ALDs), when used with or without a hearing aid, can help improve speech understanding in challenging situations like the car.