。
What an adorable little girl she is!
Literal
She [topic-は] what adorable girl [explanatory-なんでしょう].
何と + adjective + なんでしょう is a fully-formed exclamatory pattern — 'what a [adj]!' / 'how [adj]!' The 何と sets up the exclamation, the なんでしょう (the explanatory の + でしょう) softens the close into an expression of marvel rather than a flat statement. かわいらしい differs subtly from かわいい: it adds a touch of doll-like charm, often used of children or small things, and feels slightly more written or refined.