How can I use a small block of wood with a heart-shaped knot?

How small? Frame it as art?

Using a simple wood frame, add some burlap inside the frame, then glue the wood block to the burlap, positioning it so it is centered. Discard/recycle any glass from the frame. You could stain and poly the wood block first. Don't poly the back of the wood block.

Can you share a photo?

You can just put it on the wall!

I think I would try to work it into a collage of my family.

Lucky you, it sounds pretty. All the ideas above sound great. You can use it as an accent on a picture frame wall.