Sim, é possível.
Você pode seguir o sobre a proteção alfandegária e de fronteiras dos EUA para obter mais informações:
If a child (under the age of 18) is traveling with only one parent or someone who is not a parent or legal guardian, what paperwork should the adult have to indicate permission or legal authority to have that child in their care?
Due to the increasing incidents of child abductions in disputed custody cases and as possible victims of child pornography, Customs and Border Protection (CBP) strongly recommends that unless the child is accompanied by both parents, the adult have a note from the child's other parent (or, in the case of a child traveling with grandparents, uncles or aunts, sisters or brothers, friends, or in groups*, a note signed by both parents) stating "I acknowledge that my wife/husband/etc. is traveling out of the country with my son/daughter/group. He/She/They has/have my permission to do so."
Departamento de Estado tem um modelo que você pode seguir
Normalmente, eles precisam ser autenticados e, como nenhum dos pais está viajando, a companhia aérea provavelmente solicitará um dos pais.