[debono]

My younger 2009-2019 nieces and nephews (cousins kids) really stupid. When I and other elders/seniors talk to them in Mandarin Chinese, these stupid Si Ginna reply to us in English.

Even if your nieces and nephews are to communicate with you in English as opposed to Mandarin, it shows that they are fluent in English.  Mandarin if learned in school (SAP schools) will be advantageous to the student when they come out to work..............................