Parents.com has a tutorial for making heart magnets using heart shaped wood cutout and stick on magnets. The images they show are more involved however kids can have fun just painting the hearts.
Good for ages 4 and up.
Get the instructions here:
http://www.parents.com/holiday/valentines-day/crafts/easy-kid-crafts/?page=7
Candy conversation hearts are glued onto heart shaped magnets along with scrapbooking paper to make a unique valentine. Craftown shares the details.
Get the instructions here:
http://www.craftown.com/Conversation-Heart-Magnet.html