My son was 16 months old when we did this and it is one I plan to keep forever! We made this in an art class that we took.
First, he painted a piece of construction paper which I then cut into 2 flower shapes after it dried. Next, I picked a bright-colored piece of construction paper and he put a few paint strokes in different colors on there. I curled the flower petals by wrapping them around a pencil (marker, etc). Then all you have to do is glue the flowers to the background paper and your 3D Art Painting is finished!