I think it would be best if you AND your husband both went to court, and received full custody of your granddaughter. In the event something, GOD forbid, happens to you, your husband would have a lot less to deal with, concerning your granddaughter, and vice versa. You will need custody in most states to enroll her in school, take her to the Dr., have her treated for emergencies, ect., and it's much better to be on the 'safe side' and just do it right the first time.
Good luck!