。
She's as straightforward as a child.
Literal
She [topic-は] child [like-のように] is-simple.
The particle frame [noun]+のように ('like, as') turns a noun into a simile — 子供のように = 'like a child,' 鳥のように = 'like a bird.' 単純 ('simple, uncomplicated, straightforward') is double-edged: applied to a person, it can mean 'guileless, naive' (a compliment about authenticity) or 'simple-minded' (a polite jab). Context — and tone of voice — pick which one lands. Here the simile to a child leans toward the sympathetic reading.