If the child is asking to meet the donor and if the donor wants to meet the child, then I am still unsure what you are asking. You don't force them. The parents decide first and then the child is given a real choice.
How do you prepare a child for anything? You talk it out. You allow them to ask questions and you give them truthful and full answers.
When I was a child, I had a disease that meant I would likely end up in a wheelchair. At nine years of age, a doctor suggested surgery that would put me into a wheelchair for a year. This would be years before I would end up in a chair and might 'backfire', putting me in a chair years ahead of time. Hard decision for a nine year old.
So, my parents told me the truth. I was walking and would walk for 5 to 6 more years. The doctor was the best in his field and truly thought he could make me wheelchair free 'forever' if I did this. My parents had done all the research and there was a 20% chance that I would never walk again. Ever. At nine.
I was under no pressure to go ahead. My parents had decided 'yes', but they would not force me because it was a lifestyle choice, not life and death. They highly recommended that I go ahead. The doctor was concerned at doing the surgery before I was 12. I can no longer remember the reason for that. This way back in 1960, so it might not be a reason of importance by today's medical standards. BTW, the doctor went on to head the hospital -- the largest in Canada. He was a very good man. With all the information, I decided to go ahead. It was a hard and scary choice and I was in a wheelchair for 11 months. My entire school got involved because I could only go to school on the ground floor. Major life choice made by a nine year old with lots of information and help from my parents. (Total success -- great choice -- never again in a wheelchair. I still have physical problems, but they never stopped me. I can walk!)
Nine year olds can made hard decisions, but with family support and information.