。
What an adorable little girl she is!
Literal
She [topic-は] what adorable girl [conjectural-でしょう].
何と + adjective + でしょう is the softened exclamatory pattern — 'what a [adj]!' / 'how [adj]!' The plain でしょう ending here works as a gentle exclamation; layering in an explanatory ん to produce なんでしょう adds emphasis and a slightly more contemplative flavor. Both shapes feel a touch literary; in everyday casual speech, なんてかわいい子! would be more typical for the same emotion. かわいらしい differs from かわいい by adding a sense of doll-like charm, often used of children or small things.