I am such a detail person, that there is so much more I need to know.
How many blocks are you talking about? Will all of them have a heart? How big will the finished quilt be?
If only a few will have hearts, or if they are small, why not think about doing the hearts like Tartan suggested, but stuffing some of them with puffy batting so that they stand out? I would actually sew the batting into the seam so that the batting stays in place after washing. I really like the appliques that have the backs already sewn on them. When you finish attaching them to the quilt there are no raw edges to fray as the quilt gets washed later.
https://www.youtube.com/watch?v=meFsMXJeXGk
https://www.youtube.com/watch?v=djvh4Hx6ZoA
Edited to add: After watching the second video, I decided I would go ahead and cut the slit in the backing interfacing so that I don't have to worry if I were going to add the stuffing in this step. It will also insure that you don't accidently cut into the top layer of your real applique fabric. (Yes, I am the klutz that would slit the top open and have to redo the whole applique piece.) If you place the glued part where it will turn next to the batting, and then press, the batting will be glued to the backing interfacing. You can use fabric that is the same color as your applique and not worry about the glue.