。
She's nothing more than a child.
Literal
She [topic-は] mere child [nothing-more-than-にすぎない].
~にすぎない ('nothing more than, merely') downgrades the noun it attaches to — 'just a child, nothing greater than that.' Combined with the attributive ほんの ('mere, just a trifling X') the diminution is doubled: ほんの emphasizes the smallness of the category, にすぎない denies anything beyond it. Together they push hard on the 'don't expect more' framing — depending on context, this could excuse her behavior, dismiss her judgment, or warmly characterize her innocence.