Provide educational material for them to read and talk about it a lot. Do you know that ADHD is not a mental disorder? It is, in fact a chemical imbalance in the brain. Would they object to you using insulin if you were diagnosed with diabetes? In the meantime, unless you have to have their permission first, start on the meds without telling them. See if they notice any improvement in a month or two and keep a list of improvements you notice to share with them when you disclose your “experiment”. My daughter has not been supportive of me after diagnosis to the present time. She claims I use it as an excuse to be able to get away with things that she, a neuro-typical, would never think of doing or saying. I work harder at figuring out the right way to say and do as to not offend than she could ever imagine. I oftentimes fail but drag myself up off the floor and try again. That would not happen without my medication.
Good luck to you personally and with your family. It’s so easy to sit in judgement if they have never been in your brain