Here are our background and what we have achieved so far. Both my wife and I emigrated from Taiwan after 5-6th grade. We graduated from either Ivy League or other top tier undergrad and are both physicians in rural east coast city of 50,000, where the closest small to medium sized enclave of Chinese speaking immigrants is one hour away. We are fluent in Mandarin Chinese and can read most mass media publication with variable fluency, depending on the subject. I read through most of 金庸's kung fu novels in my teens. Since we hardly write Chinese these days, it does takes some effort to compose in Chinese. We speak Mandarin Chinese mostly at home.
We have two daughters who are almost 8 and 11. My elder daughter Charlotte (pseudonym) is in fifth grade and my younger daughter Georgia (pseudonym) is in third grade, a year ahead. They both enjoy playing guitar (1 year lesson so far), tennis (1 year), and painting (~ 3-5 years). In the past, they have participated in swim team, equestrian, tumbling, and piano and enjoy various camps in the summer.
Achievement in Chinese:
Charlotte speaks Chinese with us about 95% of the time while Georgia does so about 80-90% of the time. They speak Chinese with minimal or no accent and most people would think they immigrated to the US not long ago. The two of them converse in Chinese about 80-90% of the time (80% for Georgia and 90% for Charlotte). They enjoy singing and listening to Chinese pop songs on an almost daily basis. They enjoy watching 還珠格格, 犬夜叉, 真珠美人魚, and 神雕俠侶 cartoons, all in Mandarin Chinese. Charlotte can read comics in Chinese fluently and just last week started to read junior short novels in Chinese fluently WITHOUT phonetic (zhuyin). More specifically, she read 打開豹籠 and 斑羚飛渡 by 沈石溪, each in a couple of hours. So, I would say that she is solidly past third grade level in terms of reading. Georgia enjoys reading comics in Chinese without zhuyin also, recognizing about 60-70% of the words and likely still has another 1.5 to 2 years to go before she can read novels without zhuyin. Their Chinese writing is about second to third grade level.
Here is a sample of their spoken Chinese: