James didn’t know, because they hadn’t had the time to think about Rose’s guardians, because they wanted to avoid the Longbottoms of gaining Rose’s custody. He realized that they forgot to let Rose being blood-adopted by Sirius, but thankfully, the Longbottoms would never gain custody over their daughter. In case, if Sirius didn’t gain custody of Rose, the Tonks would get custody over their daughter and since the Tonks were a stabile family, the chances were high.