。
She's nothing more than a child.
Literal
She [topic-は] mere [genitive-の] child [no more than-にすぎない].
Two pieces work together. ただの＋[noun] (literally 'a mere X') downplays the noun, and the pattern ~にすぎない ('no more than X / nothing more than X') puts an even harder cap on it: 'is nothing beyond just a child.' Together they frame the assertion as a corrective — pushing back against an overestimation of her or insisting that the speaker sees through it. ~にすぎない attaches to nouns or plain-form verbs and is a common turn of phrase in arguments and essays for deflating claims.