What do I have to do for my child who was born in Australia now they have turned 10 years old?

According to the Immigration website: http://www.citizenship.gov.au/current/

Australian citizen by birth

Whether you are an Australian citizen by birth depends on the date of your birth.

Most children born in Australia before 20 August 1986 are Australian citizens by birth unless one parent was entitled to diplomatic privileges or was a consular officer of another country.

Children born after that date are only Australian citizens if at least one parent was an Australian citizen or permanent resident at the time of their birth.

Children born in Australia to parents who are not Australian citizens or permanent residents, automatically acquire Australian citizenship on their 10th birthday if they have lived most of their life in Australia.

For peace of mind and evidence you can apply for confirmation of citizenship by completing Table B of the 119 form - Application for evidence of Australian citizenship.

Note: You will need evidence of: 

Evidence of both parents’ status at the time of your birth (if available), for example a passport, and 
•     Evidence of your residence in Australia for 10 years since your birth, for example immunisation certificates, a passport, baptismal certificate, school reports, club memberships, banking records etc

Form 119 Evidence of Australian citizenship Cost as of April, 2014: $60 (Check if fees have changed on website: http://www.citizenship.gov.au/applying/fees_forms_appeals/)