。
She accidentally let her child drown.
Literal
She [topic-は] child [object-を] drown-cause-to [regret-てしまった].
A heavy sentence carrying real tragedy. 水死させる is the causative of 水死する ('to drown,' literally 'water-die') — the causative form here doesn't mean she intentionally caused the death; it can also mark allowing or failing to prevent. ~てしまう adds regret and finality — what happened, happened, with terrible consequences. The whole sentence has a stark, news-bulletin feel; it would fit a tragic report.