It depends on how your plan defines a dependent, you will have to contact your HR department. Most commonly, if the grandchild will be living with you and you will be claiming him/her as a dependent on your taxes, he/she will be an eligible dependent. You will HAVE to enroll the baby within 30 days of birth, do not be late or you will have to wait until next open enrollment.
If for some reason the grandchild will NOT be an eligible dependent, I am sure that you can get CHIP coverage through the state; if mom is a minor and in school then her income is $0 and she will definitely qualify. If she's NOT a minor or a student, then she may have to pay some premium but it is based on income.