Set the pairs aside to dry, taking apart the pieces first so that they won’t adhere to each other.
When the puzzle pieces are dry, fit them together again and show the kiddos how to paint a single heart onto the middle of the pairs, so that approximately half the heart rests on each piece. The kiddos can continue to decorate the pieces as they wish, with glitter and stickers and the other gaudy accouterments of kid-made greeting cards.
Once again, separate the pieces and let them dry. When everything is dry and set, the heart puzzle will be able to be taken apart and put together again. The kiddos can use the back side of their Valentines to write their name and some sort of horrible, punny Valentine greeting.
May I suggest “I love you to pieces?” Okay, I’m going to go vomit now.