But Nadab and Abihu died before their father, and had no children: therefore Eleazar and Ithamar executed the priest's office. Nadab and Abihu died before their father - That is, during his lifetime.
Eleazar and Ithamar executed the priest's office - These two served the office during the life of their father Aaron; after his death Eleazar succeeded in the high priesthood. And under Eli the high priest, the family of Ithamar re-entered into that office.