自己写一封信说找不到小时候的记录就好了。

Dear Sir, 

I left China more than 20 years ago.  Before I left I specifically obtained birth certification, which was recognized by USCIS when I applied for green card in 20xx and later my US citizenship in 20xx.  

In reliance on such USCIS recognization of my birth certification, I did not tell my parents to keep my Chinese residential record and school record.  After 20 years and several moves, neither I nor my parents have those secondary evidence you requested on XX/XX/2022.  

Please find attached tertiary evidence as request because none of the secondary evidence are available.  The tertiary evidences includes the following

(1) ....