Sixty years? Deng Xiaoping took power in 1978. Let's say his grandpa crossed the Pacific the moment Deng took power (impossible, as Shandong didn't enjoy the fruits of the guy's reforms until about a decade later), meaning that Ziya's dad would have been at least a teenager or a young man in order to have Ziya around ... this is post-Voldemort, so somewhere in the 90s depending on when exactly this is set. So he's more first generation ABC than second generation ABC, and the accent [i]should[/i] be a lot stronger.

Families don't westernise that rapidly, especially when in the presence of other Chinese families and Chinatowns and places where one's native culture can flourish. Fluent, unaccented English is not something you'll get just because you're born in an English-speaking country, let alone moving over as a kid. You learn it from your parents and family. If they have an accent, so will you.

My accent's still around. So's a number of other Chinese fellows I know.