How many kids are in your class??? Have you ever dealt with clay? Since you cannot have sugar or fat, you could create clay hearts. The clay is very inexpensive and you can find it at Michaels or even Walmart. It could be a craft that you would do in pieces. The first day you would give each kid a ball of clay (you would need to kind of measure it out prior) and let them flatten it. Then take a heart shaped cookie cutter - I bet they have some at the Dollar Tree or in the dollar section at Target. OR they can shape like a heart themselves without the cutter. Anyway - once they have cut out their or molded their heart then it will have to dry and harden - overnight would be fine. Then they could decorate their heart with paint, or glue and glitter, or anything inexpensive you can get your hands on. Walmart has a GREAT creative section by the fabrics!
Anyway - depending on the number of kids, this could be a very inexpensive project that could take up a few days and doesn't include candy.
Have fun!
Sam