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
Understanding Debits and Credits
Use Debtors and Creditors Adjustments
Clear the Historical Balancing Account
Suggested End Of Month Procedures
Enter Capital into the Business
Enter Government Support Payments
Enter Prepaid Expenses and Amortising Costs
Purchase a Capital Asset Under Finance
Record Debtors/Creditors Contras
Remove Duplicate Payments in Closed Period
ATO Reportable Sub-Contractor Payments
Share Utility Expenses with other Businesses