I don't want to meet my uncle's adopted child
What is the reason why you do not think you would want to meet his child?
I don't want to meet a child that has been adopted by him, therefore I'm not interested in meeting his child
It's expected that you'd want to meet your uncle's child, adopted or not. He would be really upset to hear you talk like this.
I'm close to my uncle, it's not a big deal. I'am sure he'll forgive me for what I said.
Maybe, however it is important for you to meet the child since it will be a new part of your family. It would make your uncle pretty upset to think that you are not accepting of his new child into the family.