。
She was worried about her child's health.
Literal
She [topic-は] child [genitive-の] health [subject-が] was-worrying.
心配だ ('worried, anxious') is one of those Japanese predicates where the *thing causing the worry* takes が — 子供の健康が心配だった = 'as for her, the child's health was the worry-trigger.' English flips the relationship to '[she] was worried about [the health].' This 'concerning thing as subject' pattern shows up across sensation predicates: ~が好き ('like'), ~が必要 ('need'), ~が心配 ('worry'), ~が怖い ('scared of') — the experiencer is the topic, the thing experienced is が-marked.