Since everything that we see is light that enters the eye through the pupil, which is really very small, children can experiment with
1. different sizes of holes
2. different distances of the hole from the eye
Holes can be cut out from pieces of cardboard, such as cardboard that comes with shirts from the laundry, one hole to each piece. A pinprick can serve as one hole on one piece of cardboard. Other holes can be 1/4 inch across, one inch across, and two inches across (maybe other sizes, too).
Children can record their findings using two factors: size of hole and distance from the eye.