。
She can't stand being treated like a child.
Literal
She [topic-は] child [like-のように] is-treated [nominalized-こと] [target-には] cannot-endure.
A dense little sentence stacking three things: passive 取り扱われる ('be treated/handled'), nominalizer こと turning the passive clause into 'the situation of being treated like a child,' and the contrastive-target には marking what specifically can't be endured. 我慢ならない is a slightly elevated negative meaning 'cannot bear, cannot tolerate' — note the irregular ない attached to the verb stem 我慢なる (an old form), giving us the fixed expression ~には我慢ならない for 'I cannot stand X.'