Plan to visit China in mid August 2023 for 2 weeks (destinations are Guangzhou & Hong Kong)

4 adults holding British passport & 2 adults holding Malaysia passport
2 children aged 5 & 2 holding British passport
Do we have to buy the return air tickets first as proof before we apply for the entry visa
Do the children need to turn up in person at the visa office on the day of application
Passport fee for UK citizens is £151 & Malaysian £86