Children travelling on Mother's passport

Hi, My wife and I along with our 02 daughters (age 07 & 11) intend to stop in Hong Kong for a couple of days on our way back from Australia in October. my wife & I hold Sri Lankan passports. My 02 daughters are on my wife's passport and I wish to know if we need to submit 04 applications for each of us or 02 applications for the 02 passports?