You may have been misinformed---depending on how you were getting the 2nd stimulus amount-----if you are getting it as a recovery rebate credit on your 2020 tax return then that credit can be used to pay debts you owe including back tax or child support.   If the 2nd stimulus check was sent to you separately----ahead of when you filed your 2020 tax return, then it could not be seized for child support.   But the recovery rebate credit is subject to being seized.

There is no place on your 2020 tax return to enter anything about your 3rd stimulus check.   That is sent separately by the IRS and has nothing to do with your 2020 tax return or the recovery rebate credit.

TurboTax cannot track your 3rd stimulus or tell you when it will arrive.  The IRS is issuing 3rd stimulus checks on its own timetable.

You can use the IRS Get My Payment site to see what is expected for your 3rd stimulus check:

https://www.irs.gov/coronavirus/get-my-payment

https://www.irs.gov/coronavirus/get-my-payment-frequently-asked-questions

The first and second stimulus payments paid in ADVANCE were safe from child support offsets however if you got the missing payments on the 2020 return as a credit  it could be taken.  

Once the 2020 return is processed the third stimulus advance payment will be released if you are eligible based off the income on the 2020 return.  https://www.cnet.com/personal-finance/your-third-stimulus-check-can-be-seized-heres-what-to-know/