。
She's just a child.
Literal
She [topic-は] mere child is.
ほんの ('mere, just') stripped down to a simple polite copula です gives the cleanest possible 'just a child' framing — softer and more matter-of-fact than the にすぎない version, and pitched at conversational politeness. ほんの always attaches directly to a noun (ほんの少し 'just a little,' ほんの数日 'only a few days,' ほんの冗談 'just a joke') — it doesn't conjugate or stand alone.