The children were terrified by the violence of the storm.
Literal
Storm [possessive-の] fierceness [to-に] children [topic-は] feared trembled.
激しさ (fierceness/intensity) is the noun form of 激しい. 恐れおののく (to tremble with fear) is a literary compound: 恐れる (to fear) + おののく (to tremble). The compound verb is more vivid and dramatic than either part alone.