Post token audit, there is an issue with child's contract with respect to potentially locking up ether and not being able to withdraw it. Are you aware of this or how can we resolve this issue?

The short answer is there is no problem with that contract.

The long answer is that function is intended to be payable and without a withdraw function because it enables meta transactions which are a way to send gasless transactions in Ethereum(and polygon). Certik (or any audit platform) most likely used an automated auditing tool that detected a pattern and expected it to be a vulnerability.

If you do not know about meta-transactions, I recommend you check out this doc from Polygon: https://docs.polygon.technology/docs/develop/meta-transactions/meta-transactions