In the story I'm working on, there are two little boys, both around the age of eight or nine. One is a native Chinese speaker with no English, the other is a native English speaker with no Chinese. They meet fighting off bullies and end up becoming friends, and end up teaching each other how to speak in their respective languages (...with lots of misunderstanding and fights in the interim, but naturally picking up the bad words first). They're living in an English-speaking area, so the Chinese kid would learn English faster, but the Chinese kid's mom only speaks Chinese, so when they are at that house, which they are most of the time, they only speak Chinese--so there's almost full-emersion going on with both of them. When they're alone, though, they speak a mix of Chinese and English depending on the comprehension-level and/or "don't wanna speak that stupid language no more!" level of the person speaking.
Background over, question one: roughly how long would it take for the native English speaking kid to have an intermediate-level/lower-advanced grasp of spoken Chinese (for an eight year old, so it's not the same amount of vocab or grammar as an adult)? I'm thinking hearing it and using it every day, the kid would be speaking pretty well after four or five months, but I have no idea.
Question two: What are affectionate diminuatives for children in Chinese? In English, we use nicknames and often add a "-y" sound to the end of names, in Japanese they add "-chan", and in Russian you've got "-ska" and "skaya" thing, but what would you say in Chinese? "Xiao [insert kid's name here]"?