SAN DIEGO — Reports of unaccompanied migrant children being forced to stay overnight in parked buses at the Dallas convention center are “completely unacceptable” if true, U.S. Health and Human Services Secretary Xavier Becerra said Friday. Dr. Amy Cohen, a psychiatrist and executive director of the advocacy group Every Last One, said a 15-year-old Honduran boy was held on a bus from Saturday to Wednesday, using the bus bathroom during that time and unable to move about freely or communicate with his family. The boy encountered at least three other children who were held as long in the parking lot of the Kay Bailey Hutchison Convention Center, said Cohen, who has been in contact with another child who was confined to a bus for an extended period.