We have two kids, a toddler and a preschooler. Before our preschooler was born, we pledged that we would make an effort to ensure she grew up being able to speak both English and Mandarin, to prepare her for a more internationally connected world, and to ensure that she can talk with her relatives in Taiwan.
For the first two and a half years of her life, she was exposed to Mandarin most of the time. At home, my wife spoke to her in Mandarin in 99% of their conversations, and myself, about 90% of the time. We read Chinese children's books to her, explained concepts to her in Mandarin, and showed her Mandarin VCDs and DVDs. She didn't attend daycare, and only had occasional contact with English-only speakers -- neighbors, my relatives, and a few kids living nearby.
The immersion strategy seemed to work. When she was 19 months and 25 days old, my wife and I sat down and recorded all of the words she was able to speak in both languages. Here's the Chinese list (translated into English):
duck, cow, horse, dog, cat, crow, bird, squirrel, mouse, pig, elephant, fish, turtle, ant, bear, monkey, rabbit, sheep (plus sounds they make)
telephone, key, toy, car, boat, book, door, baloon, shoes, socks, clothes, pants, belt, glasses, watch, hat, light, fridge, swing, candle, crayon, baby, diaper, bao bao (抱抱), kiss, bike
grapes, candy, vitamin, ice cream, noodles, rice, meat, egg, dofu, melon, banana, blueberries, cherry, strawberry, eat, bread, water, mushroom, soup, cookie, knife, fork, spoon, button, barette, air plane, music, bag, phone, fan, tea, beer, tomato, beans.
ear, nose, mouth, neck, knee, hands, tummy, belly button, hair, teeth, chin, feet, poop, pee
hot, cold, stinky
moon, sun, tree, cloud, rock, rain, stars, leaf, thunder, flower
eat, look, want/don't want, hurts, take a walk, swim, wash, cry, laugh, sleep, paint, open, close
mom, dad, grandpa, grandma, aunt (maternal & paternal), uncle (maternal)
thank you
Don't ask why she knew how to say beer (啤酒). Here's her English vocabulary list at the same point in time:
bird, bee, butterfly, boat, key, elephant, powder, teddy bear, juice, rui rui (raisins), ice cream, ye-you (yogurt), juice, bi-doom (baloon), soup, swing, slide, flower, pocket, telephone, uppie, big hug
delicious, beautiful, pretty, hot, cold, yuck-yuck, yum,
thank you, pleesh (please)
... plus the proper names of a few neighbors, as well as my parents' cats, and TV characters including Arthur, Barney, Elmo, and all four Teletubbies. Neither the Mandarin nor English lists include words that she understood, but could not say.
The Mandarin dominance began to shift very shortly after she started English-language preschool. Besides beefing up her English vocabulary, it also resulted in her asking me not to read stories to her in Chinese anymore and a newfound interest in English-language TV. By the time she turned four I'd say her Mandarin vocabulary was not as good as her English vocabulary, but her Mandarin grammar was definitely better than her English grammar, which often skipped articles and correct verb usage compared to her classmates. Sometimes Mandarin syntax will creep into her English conversation -- for instance, she'll say "open the light" instead of "turn on the light", which she gets from Mandarin (開燈).
We've tried to keep her Mandarin skills up to par as best we can. We subscribed to a satellite TV package which includes "Yoyo TV", a very good children's channel from Taiwan. We enrolled her in the local Chinese Community Center school, which meets every weekend, but she complains that "there aren't any toys there" like the English-language preschool. She still speaks Mandarin about 50% of the time with my wife, but never with me -- if I ask her a question in Chinese she'll answer in English.
Today, we took heart in the fact that she spoke entirely in Mandarin with a little friend who came over whose parents are from China, but I have a feeling that these types of exchanges will fade as both get older and enter the language and social atmospheres of the local public school system. We've thought about Chinese summer camp, or summers with the in-laws in Taiwan when she gets older, but judging by the experiences of my adult friends who grew up as ABCs or CBCs (American- and Canadian-born Chinese) it's very difficult to remain a proficient Mandarin speaker unless you spend lots of time in Taiwan or China.
As for our son, he is just learning how to talk, but we'll undoubtedly have an even tougher time with teaching him Mandarin. Why? The person he interacts with most during the day (aside from my wife) is his older sister, who speaks with him exclusively in English. This is a factor our daughter didn't have as an influence when she was his age. But we'll try pushing Mandarin with our son -- in the long run, even if he doesn't learn how to speak Mandarin properly, planting a nugget of the language in his brain will serve him well when he sees his relatives, explores his roots, develops an interest in doing work associated with Taiwan or China, or meets someone from either country.