A child whose parents have died is called an orphan.
Literal
Parents [subject-が] died child [topic-は] orphan [quotative-と] is-called.
両親が亡くなった子供 is a relative clause: 'a child whose parents died.' ~と呼ばれる (to be called X) uses the quotative と + passive of 呼ぶ. 孤児 (こじ, orphan) is a formal term. A definitional sentence.