"Sweet innocence" describes someone or something that is pure, innocent, and gentle, often associated with a kind and pure-hearted nature. Full definition
[Diaz] can blunder into the crudest situations with such sweet innocence as to inspire giggles rather than groans. (rottentomatoes.com)
Shining above it all, like a kewpie - doll saint, is Drew Barrymore — whose sweet innocence and sexy romanticism have survived movies as bad as this before (metacritic.com)
«I'm afraid of a life without the sweet innocence that I see reflected back in my daughter's big blue eyes. (babygaga.com)