Single parent traveling with child ' What docs needed?

#1
I´m returning to the states with my child. We are both from there and her dad is there. I understand from the US Embassy site, the Embassy here in Bs As and UAL that I need an authorization letter signed by her dad and notarized by the Argentine Consular´s office in the states to leave. Her dad, is Argentino and has traveled here with his éx's son and never was asked for any letter. Has anyone had any experience with this? Does anyone know really how essential it is? Ap`parently the consular´s office is in LA and her dad is in SF ' not very convenient. Any help is greatly appreciated.