Hi Kim.  To put a child on the Airport Watch List, you need to file an application to the Federal Circuit Court (usually) for interim and final parenting orders.  Include as an interim order a Watch List order which is typically worded "That until further order the [Father/Mother] [full name and date of birth], and [his/her] servants or agents be and are hereby restrained from removing or attempting to remove or causing or permitting the removal of the [child/children] from the Commonwealth of Australia AND IT IS REQUESTED that the Australian Federal Police give effect to this order by placing the [name/names] of the said [child/children] on the Watch List in force at all points of arrival and departure by air or sea in the Commonwealth of Australia and maintain the [child's/children's] [name/names] on the Watch List until the Court orders its removal".

You will need to file an affidavit in support.  Once you file, you send a copy of your Court sealed application to the AFP.  Their website will tell you what to include.  The Alert will remain until it is discharged by the Court.  Sending a copy of your sealed application to the AFP will place the child on the Watch List immediately.  You do not have to wait to obtain an order from the court.

If a child does not have a passport (or it has expired), it would be a good idea to lodge a Department of Foreign Affairs and Trade's Child Alert Request form to stop a party from applying for a child's passport without your consent.  This form can be lodged at any Australian Passport Office or sent to Passport Operations (PC9), GPO Box 9807 in your nearest capital city.  A parent or person with parental responsibility for a child is entitled to do this.  It is actioned immediately and will remain in force for 12 months or until the child turns 18 or a court order discharges it. The form is available at passport offices.  You would need to include with the form a certified copy of any existing parenting order or a statement that confirms that you are entitled to make the request.  You should also include a statement to explain why you are requesting a child alert. 

Remember also that if court proceedings are on foot, the court does have the power under section 67ZD of the Family Law Act "where there is a possibility or threat that a child may be removed from Australia" to order the passport of the child to be delivered to the court.