Both should be entered, however, because the child qualified for Medicaid, that disqualifies the child's portion from the Premium Tax Credit.

I think you need to enter $0 for column B of the child's 1095-A, and make sure the amount for column B of the mom's 1095-A is only reflecting the amount for her (not her and her child).  You should be able to look that up on the Covered CA website (if the number is column B is close to, or less than the amount in column A, it is presumably properly only showing the mom).