The best way to bill for a baby is to bill using mom's PHN. It often takes some time for the baby to become enrolled in MSP so we find claims are paid quicker by billing under mom.
To do this you will create a patient using baby's name, mom's PHN, and mom's date of birth. Within the claim creation view you will see a check box titled Bill Child. Please select this and provide the baby's date of birth next to it. This will correctly bill for baby under mom's PHN.