The War that Saved My Life by Kimberly Brubaker Bradley

Could be a long time, then. I doubted I’d ever go to school and wear a
birthday hat, but still— “Will we have cake for tea on my birthday? On the
day you told the man?”
“Yes,” Miss Smith said. A sudden look of sadness washed over her face,
then disappeared so quickly that if I hadn’t been looking right at her, I never
would have seen it. Sadness? I thought. How did I know that was sadness?
And why would Miss Smith be sad?
“That was Becky’s birthday,” Miss Smith said. “It’ll be nice to have a
reason to celebrate the day again.”
“That’s a lie,” I said. I wasn’t angry about it, but it was one.
“Oh.” Miss Smith forced a laugh. “It is and it isn’t. It will be hard for me,
but I’d like very much to be happy again.”