See https://www.united.com/web/en-US/con...s/default.aspxWhen children and adults are traveling together on separate reservations
When children and adults are traveling together on separate reservations, the unaccompanied minor policy does not apply. Before the day of travel, call the
United Customer Contact Center so a United representative can add the relevant information to each reservation. Please keep in mind that children traveling on a separate reservation will need to check in with a United representative at the airport, as online and mobile check-in aren’t available for children under 16 years old.