Even this thread has made me cry. Mine are:
Someday by Alison McGhee. Just a beautiful story about loving a daughter, and her hopes for that daughter's future (but things like 'someday you will dive into the clear, cool water of a lake' rather than aspirational stuff) and then at the end it talks about how when her daughter is old... 'You'll think of me, love' (fuck it I'm off again!)
Wherever you are, my love will find you. Just a beautiful love poem to a child saying that they're never alone. 'You are my dearest, my darling, my star. And my love will find you wherever you are'.
The invisible string. This is a mum explaining that we're all connected to the people we love with an invisible string. When you feel a tug on your heart it's because you're missing them and it's making a tug on the string from your heart to theirs. It can go all around the world, even up to heaven and it never breaks no matter what.
Plus loads of others mentioned on this thread - paper dolls, stickman, Michael Rosen's sad book (that one is so painful). And I sobbed at The Boy in the Striped Pyjamas. Also Maus by Art Spiegelman (but these were painful, horrified sobs not painful beautiful sobs like some of the other books).
By the way, in case 'Love You Forever' by Robert Munsch didn't make you cry enough already - did you know that he wrote that four line verse (I'll love you forever, I'll like you for always, as long as I'm living my baby you'll be) after both his first and second babies were stillborn? His wife wasn't able to conceive after that and they adopted children but the story came much later than those painful lines.