Minor kids with a minor child in the state of tx

I have a 16 yr old son that has a daughter by a 16 yr old girl. Does the girls mother have any say as to when and where my son takes his daughter, She is only 9 days old and the girls mother is stating that she is legally responsible for the daughter of my son. I talked to the office of the attorney general and they stated that the grandmother has no say, that the decisions are made between the mother and father although they are minors. The problem we are having is that the other grandmother is not allowing my son to take his daughter to his house where I the mother of the boy and his grandparents are here.. Plz help...