It is difficult to determine the cause of hearing loss. Sensorineural hearing loss is caused by issues in the inner ear or cochlea. Conductive hearing loss is caused by problems in the middle ear. If sensorineural hearing loss has been identified, it's possible there is a genetic link. Are there people in your genetic line who have or have had hearing loss?

I'm glad you asked about prevention. The most common cause of hearing loss is over exposure to noise. Noise is everywhere in our environment, but exposure can be limited by avoiding it and by using well fit ear plugs when that's not possible. Things like loud music, noise at large sporting events, fireworks, gun shots, noisy machinery, etc. are problematic. Sudden extreme noise and lesser noise experienced over time can both damage hearing. Best to avoid exposure is possible, and to use ear protection if not.

Anything over 90 decibels can damage hearing. Again, duration matters. You can download a decibel meter on your cell phone. It's interesting to see how noisy the environment can be. Here is a link to what audiologists refer to as 'the speech banana'. It shows where typical hearing lies on an audiogram, and shows some common sounds that are dangerous.
https://www.ndsd.nd.gov/sites/www/files/documents/Presentations/Audiogram-Sp-Banana.pdf
It's important to understand that some people seem to be less prone to noise induced hearing loss than others are.

Was your granddaughter born prematurely? In some cases hearing can improve after that initial infant testing. Your granddaughter should be retested regularly.