If at first you don't succeed, have realistic expectations
We have a house rule with our kids. Ever since they were of speaking age, we would not respond to them unless they spoke to us in Chinese. However, I do try my best to be less of a tiger mum in this approach. I consider my battle half won if they begin any conversations with me in Mandarin without me reminding them. I would encourage my children to try to speak as much as possible in Chinese and to use English where necessary for words I know that they have not learnt or expressions that they struggle to convey in Chinese. I would then show them how to say the phrase or word in Chinese and have them repeat it.
I find this method more encouraging as it is less frustrating for them than having to struggle desperately to find the right word whilst they are trying to tell me about something exciting that happened at school. In addition, they will most likely remember the new vocabulary because there was a practical purpose to know the word as opposed to something that they had to memorise as part of a lesson. It is also worth remembering that if your children are learning Chinese as a second language in a society where for example, English is the dominant language, they will naturally have a limited exposure to Chinese. As such, it isn’t really fair for us to expect them to be able to communicate fluently in Chinese all the time.
One of my favourite ways to develop vocabulary is to allow them to watch their favourite cartoons in Chinese or play Chinese game apps (YouTube is a good resource). This way they become fully immersed in the language and enjoy themselves whilst doing so. Click here for a selection of our favourite cartoons in Mandarin available to watch online for free. You can also browse our shop for a selection of DVDs (Language options include Mandarin and Cantonese).
You could also consider learning the language with them. I too sometimes struggle to speak to them fluently in Chinese as a non-native speaker as I do not have the vast vocabulary of a native speaker to express my exact thoughts, especially now that they are getting older and our conversations are becoming more mature. We would occasionally look up Pleco (an online dictionary) for words or phrases that we do not know and learn how to say and repeat it in Chinese together. It also demonstrates to your children that learning is an ongoing process for young and old alike.