Question: I am translating a text from Russian. I need to render an expression that originally says something like "she flung her eyes open even wider" - but I am not sure this is the right English usage. Is it? And if it is not, what us the best way to convey the meaning?

She could just "fling her eyes open" if they were shut or ajar. But they were already open. "Widened her eyes" seems to lose the atmosphere.

(For those who understand Russian: the original is "она распахнула глаза ещё шире").

Answer: 
In this instance it is a modest way to acknowledge and accept a compliment.
When the girl praises the boy for asking permission, the boy is trying to say "I"m not always so dutiful. This instance is actually a bit of an exception to my normal behavior". It was a "good day", presumably on a "normal" day, he would have behaved differently.