Let's review payroll math:

Gross wages plus employer taxes = net take home and all payroll checks written

"and then enter the checks into the register using same subaccount for the child support check "

Which Register? The Bank?

"I enter payroll as a journal entry with child support as a subaccount of employee wages "

That is an error. The journal entry will be Debit Gross Wages, and Credit "Child Support Liability account."

When you write the check to pay the garnishment, on the Expenses tab, you list the Child Support Liability account.

Garnshiments NEVER hit the P&L. This is NOT the employer's garnishment or expense. It is a liability: you held back money from the employee's takehome check and need to prove you paid it out to the child support enforcement division.

It is Balance Sheet activity, not P&L.