My heart goes out to you.....such a sad story. You must try not to feel guilty though, as you are doing your best. Your Mum needs a proper medical assessment, with an appropriate care package put in place, so that you and your Dad can have some respite. She is obviously very depressed about her situation, which is understandable, but you should not have to take on all the responsibility yourself. Do you live with your parents? Also, are there any other family members, apart from your Dad, who could help? There must be some specialist in the medical profession who can offer some advice about her reaction to drugs. You may have to pester your GP and insist on getting an appointment with a consultant. Otherwise your own health is going to suffer and your 5yr. old will end up suffering too.
I know how hard it is to see your parents go downhill and you want to do everything you can to improve their quality of life. But there comes a point when you need to reach out for expert help and advice and I would suggest your first port of call is your own GP. Ask for a longer appointment than the 10min. slot, when you ring and make a list of all the things you want to say and questions you need answering.
It may take a bit of time to find out, but there are many people who could help you.......social services, occupational therapists, McMillan nurses, PD nurse, Parkinson's social worker and I'm sure there are others. It's worth making enquiries and finding all the help you can get.
I do hope you find some solutions to your problems and that your Mum can come to understand what a loving daughter she has.