There is the hint of a smile at that, because yes, Cosette likes him to eat. For a moment he is lost in the memory of her insisting that she would eat black bread like him, and so making him choose white.

He eats a bite, and the smile fades away.

'But you say you have read the book, monsieur. You know I am not her father.'

What is the use of trying to pretend it is so when they both know it is not?