Eva Ibbotson: Madensky Square
The main character, Susanna, has gone to reclaim her 5 year old daughter who was adopted the day after she was born (long story).
However, she sees the child happily playing in the garden with her adoptive mother bringing her milk and biscuits. An idyllic scene. The adoptive mother sees Susanna and realises she's her daughter's natural mother. She stands there in shock and Susanna quietly turns and walks away, leaving her daughter in her happy home with her happy family:
"I was being good, you see. And as a matter of fact, it nearly killed me."
It's so understated, yet gives me shivers every time I read it.