Please share your experience related to non-US citizen tax return filing for CTC.

I have a husband/wife with ITIN; children have SSN. Taxpayers want to file a tax return as they are eligible for CTC.  For example, taxpayer is telling me they have $30k income paid off the books. Taxpayer has no proof of income. On one hand, I am reluctant to prepare this return not having proof of income. On the other hand, the client is trying to do the right thing and pay his share of fed, state and local tax, but, of course, he also wants the CTC. 

How are other preparers handling this situation?  

Thank you for your input!