。
She felt the need to get her children used to waking up early.
Literal
She [topic-は] children [object-を] early-rising [to-に] make-accustom necessity [object-を] felt.
慣らす ('to accustom, to habituate, to break in') is the transitive partner of 慣れる ('to become accustomed'). Same transitive/intransitive logic as so many Japanese verb pairs: 慣らす acts on someone, 慣れる describes the resulting state. ~に慣らす marks what one is being accustomed to. 必要を感じる ('feel the need to') is a somewhat formal way to express recognition of necessity — 必要 ('necessity') + 感じる ('feel'). 早起き ('early rising') is the noun form of the compound verb 早起きする.