The title character of Edmond Rostand's Cyrano de Bergerac is in love with his cousin Roxane whom he used to play with as a kid. At Act II Scene VI, Cyrano and Roxane tenderly remember when they were children and played each spring at Cyrano's house at Bergerac.
Roxane: Confession next!... But, ere I make my shrift, You must be once again that brother-friend. With whom I used to play by the lake-side!... Cyrano: Ay, you would come each spring to Bergerac! Roxane: Mind you the reeds you cut to make your swords?... Cyrano: While you wove corn-straw plaits for your dolls' hair! Roxane: Those were the days of games!... Cyrano: And blackberries!... Roxane: In those days you did everything I bid!... Cyrano: Roxane, in her short frock, was Madeleine... Roxane: Was I fair then? Cyrano: You were not ill to see!
Éponine Thénardier could be seen to be this in the musical of Les Misérables. However, in the book, Marius doesn't meet her until adulthood and barely knows her.
The traditional Pantomime version of Cinderellaadds the character of Buttons, who's been a servant to the family since both he and Cinders were children, and is in love with her. She is destined to marry the Prince. Depending on the production, she either never notices how Buttons feels or lets him down gently.
Patience and Archibald in Gilbert and Sullivan's Patience were friends as small children, but then lost contact with each other for a while before falling in love during the play.
Motel and Tzeitel in Fiddler on the Roof. Initially it seemed to be a dead end because the local matchmaker set Tzeitel up to marry the town's wealthiest citizen, but the pair eventually convince Tzeitel's father, Tevye, to let her marry Motel instead.
In the Rodgers and Hammerstein musical Allegro, Jenny is Joe Taylor's girl from childhood, and she keeps writing to him while he is away at medical school. When they meet again, Joe decides to marry her even before finishing his education, though Jenny would prefer it if he were wealthier. In the second act, they move to the city at her insistence so he can earn more money at a big city practice. When Joe hears that his wife is having an affair with a soap tycoon, he feels he doesn't care anymore about the girl he's known since he was eight years old.
Stuart Patterson's play Cinderella, which is based on the panto but is not itself a panto, calls the Buttons character (see above) Callum, and he and Cinderella end up together once she realises the Prince is a Prince Charmless. Callum then turns out to be Lost Orphaned Royalty of a neighbouring kingdom.
In Dorothy L. Sayers' The Emperor Constantine, Maximian jovially talks of how Fausta and Constantine were sweethearts as children. He's about to propose a political marriage — one that requires Constantine to divorce his wife and his son's mother.