As the primary applicant, you are eligible to add family members like your spouse, de facto partner or children to most of the visas. Your children will be eligible to be added to your visa as long as they did not reach age 18, they are still living with you and are financially dependent on you. If the child is older than 18, but is dependent on the parent and cannot work due to physical disability or mental disability s/he can still be added to your visa. For other visa types, like visitor visa or working holiday visa, there is no option to add a secondary applicant.