The child's household registration is in Mianzhu, and the social security card is handled in Mianyang. Where is the insured place after payment?

According to the policy I understand, the place where social security contributions are insured is generally determined according to the location of household registration. In the situation you described, if the child's household registration is in Mianzhu, then the insured place is probably Mianzhu. Because the location of the household registration is usually the key factor to determine the location of social security payment. But the most accurate answer also requires you to consult the local social security department or relevant government departments to obtain specific laws and policies.