You don't mention what your young cousin believes. That's pretty important; if she believes what her parents do, you may offend and alienate her if you're not careful.
I don't really know, but I suspect that she believes in God with the same sincerity that an 8-year-old believes in Santa Claus... and that she also doesn't know that she has atheist relatives.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.