My family will be travelling to China for a holiday and then onto Hong Kong

When in Hong Kong, we would like to do a day trip to Shenzhen.Do we need a double entry visa for this day trip?
We are Australian Nationals and need to apply for our China holiday visa so do we get a double entry visa in Australia before our departure?
Also my son will be joining us in Hong Kong only. What visa requirements does he need for the day trip to SZ please?
Thank You for your assistance.
Bronwyn
Australia