You can start as early as Year 7 or 8. We will learn all the vocabulary and grammar needed to pass the exam. During the last year we will do lots of practice using past papers, including listening, reading, writing and speaking.
If you aim for Chinese Proficiency Test (HSK 汉语水平考试), I use HSK Standard Course (official book for exam preparation) and previous exams papers. If you want, we can include the speaking part (HSKK).
If your younger ones want to learn something new, why not try Chinese? My kids classes are fun and easy and usually last for 30 minutes where we use toys and games to help us learn the language.
Nowadays lots of people want to learn a new foreign language. Chinese is a very good choice as it is entirely different from English and helps us train the right hemisphere of the brain. It is up to you to decide if you want to learn the characters or just need some speaking and understanding practice.
If you have a specific request, I can provide you with a course set according to your needs, level and goal.