Create a Liability GL Account called Child Support (or similar).
When entering the Payroll Journal, add the child support as a credit. This makes up the child support deduction.
When paying the child support to a third party, enter a chequebook payment from the bank account to the liability account. This will net off the child support GL account.
Further information
Clear the Historical Balancing Account
Suggested End Of Month Procedures
Enter Government Support Payments
Purchase a Capital Asset Under Finance
Record Debtors/Creditors Contras
ATO Reportable Sub-Contractor Payments
Share Utility Expenses with other Businesses
Understanding Debits and Credits