Both parents have died — what will become of those children?
Literal
Parents both [completely-に] died-regrettably those children [topic-は] what-happen [wonder-んだろう].
死んでしまって uses ~てしまう for the finality and sadness of death. どうなるんだろう (what will happen / what will become of them) combines どうなる (what happens) with んだろう (I wonder). The sentence expresses concern for orphaned children.