A hearing aid makes sound louder. That works when the inner ear can still pass sound on. In severe to profound hearing loss, many of the inner ear's hair cells no longer work, and making sound louder does not make speech clearer.
A cochlear implant goes around the damaged part. A surgeon places a thin electrode inside the cochlea, the snail-shaped hearing organ. An outer processor sits behind the ear, picks up sound and sends it to the electrode, which stimulates the hearing nerve directly. The brain then has to learn what those new signals mean. That learning is what rehabilitation is for.
The decision is made by the implant team at the hospital, after hearing tests, scans and a medical check. Our part is to give that team a clear, careful picture of how your child hears, with and without hearing aids. If your child is still at the testing stage, see child hearing tests for OAE, BERA and play audiometry.