I am 76 and have PD 5 years and have 15 grandchildren from age 2.5 months to 32. I discuss it freely with my college age & up grandchildren. I do not discuss it with the 8 younger than that. I feel like the parents would know better than me how their individual child would handle any information. Their parents are a primary care physician, a OB/GYN, and a Veterinarian. I am sure they would each have strong opinions about what they would want to hear.

I told my children and grandchildren. (Then aged 8and10) on the day of diagnosis. My eldest daughter had helped me obtain a lot of information in advance as my GP had already said he thought this is what I had. They were very pragmatic and supportive and still are and when I just got on with life they accepted that change would be slow and worrying would not help. They have grown up knowing that nan does some odd things now and gets very emotional but they seem to take it in their stride and help if I ask for it( which isn't often ) and accept as the norm that sometimes nan stands up to eat. Children take their lead from your actions and attitude and are far more resilient than we give them credit for.

When I was diagnosed we decided my wife and I not to tell my mother in law in order to protect her. Very bad idea ! She detected rapidly that something wrong was going on So we had to tell her and she could accept it. Everything Is easier when you trust your family

https://www.parkinsons.org.uk/sites/default/fil...

I thought this was wonderful. I have a six year old granddaughter who is asking a lot of empathetic questions.

You might want to Google “children’s books on Pd...there are a bunch but I don’t know if any will apply to your older grand child. I haven’t had to discuss this yet but I would start with a family meeting...GPs, Ps and kids and have the GKs lead the discussion
Best of luck!