。
She's still just a child, you know.
Literal
[topic-は] still mere child [copula-です] [you-know-よ].
ほんの ('mere, just, only') is an attributive adjective minimizing the noun it modifies — ほんの子供 ('a mere child'), ほんの少し ('just a little'), ほんの数分 ('only a few minutes'). Stacks naturally with まだ for double emphasis on the smallness/youth. Sentence-final よ marks the assertion as new information for the listener — 'I'm telling you, just so you know.' The whole sentence functions as a defense or excuse — said in protective response to criticism of a young person, or to recalibrate expectations downward.