Once a child Sim has reached a good level of friendship with another child, a notification may arrive letting you know that they have a crush. They can decide to keep it secret, OR they can decide to declare their love by asking their crush if they want to become sweethearts. The outcome of this interaction can be: negative (the crush doesn't accept and your Sim will be sad for 1 day, then they will no longer have a crush); positive (the crush agrees to become sweethearts).
Other Interactions:
-Between kids: Ask if they have a crush; Make a list of the nicest kids in school; Talk about the crush (if they have one)
-Between child & older sibling: Ask how is like to have a crush; Reveal to having a crush (if they have one)
-Between sweethearts: Give a kiss; Talk about getting married when adults; Give a loving hug; Promise to always be together; Break up with an invented excuse.
-When growed up (from teen and above): if two Sims have been togheter while kids, they can "Remember when they were sweethearts"
Bonus Interaction:
At my daughter's request, child Sims will be able to write about their crush/sweetheart in their secret journal. ^^
This interaction requires you to have GP Parenthood installed!
If you don't have it, just delete the "REMOVE if you don't own Parenthood" folder.